We preview each of the quarterfinals at the Olympics, the gateway to the medal matches.

Federer vs. Isner:  Unable to claim a medal in his three previous appearances at the Games, the top seed fell a round before the medal matches to an American in Beijing.  Tasked with a similar assignment here, Federer faces a challenge more overtly sinister than Blake in Isner’s monumental serve.  The American has not dropped a set through three victories that culminated in a minor upset over the eighth-seeded Tipsarevic, ending with a 30-point tiebreak.  Considering the surface and the quality of the serves on display, another tiebreak or two should develop at the venue where Federer has lost only three times since 2002.  On two of those occasions, opponents with heavy serves used that single shot to suffocate the Swiss champion, and Isner echoed their exploits this February.  Overcoming Federer in Davis Cup, the American succumbed to him in the Indian Wells final after an ominous start, much as he did five years ago at the US Open.  After an unsteady opening match, the world #1 appears to have found his groove in a fashion similar to his fortunes at Wimbledon Part I.  In general, he has found ways to deflate one-dimensional serving machines in tense matches, as players like Karlovic and Raonic can attest.


Nishikori vs. Del Potro:  Of scant interest to fans from neither Japan nor Argentina is this quarterfinal, if only because the winner would seem to harbor so thoroughly overmatched against Federer in the next round.  Reaching the semifinals itself qualifies a contender for a potential bronze medal, however, so Nishikori and Del Potro do not compete for a meaningless distinction as they would at a conventional tournament.  The Japanese star has become arguably the most surprising story of the week, especially when he ground his way past the ultimate ATP grinder in Ferrer.  Overlooked by most observers, the former US Open champion avenged a loss at that tournament to Simon in a three-setter that resembled Nishikori’s victory.  Outside the obvious height advantage and the serving superiority that it creates, Del Potro owns a clear experience edge with his extensive appearances deep in elite draws.  But Nishikori remains superior in footwork and movement, two strengths underrated on grass.  He also seems mentally more resilient than the Argentine, although that dimension will matter only if he can stay within range on the scoreboard to create pressure on the heavier hitter across the net.


Almagro vs. Murray:  A month ago, Almagro might not even have ranked third on the list of Spaniards most likely to advance within a victory of the medal rounds.  The last remaining member of this proud tennis nation in either singles draw, he advanced past the undistinguished trio of Troicki, Bogomolov, and Darcis.  No Olympics quarterfinal appearance comes entirely by accident, though, so Murray cannot afford to bring complacency to a match that he will enter as the heavy favorite.  Extended to three sets by Baghdatis, the home hope reverted to his familiar habit of subjecting British onlookers to cardiac arrest at the All England Club.  Although Almagro seems far less dangerous on grass, he possesses an explosive first serve and seldom declines to strike his groundstrokes with authority.  Less useful here are his elongated swings and one-handed backhand, a less compact and less reliable stroke than Murray’s two-hander.  Unless his own first serve abandons him, the Scot should exploit Almagro’s ebbs and flows to reach the medal rounds routinely with plenty of energy in reserve for a more intimidating foe. 


Tsonga vs. Djokovic:  The men’s match of the day at first glance, this encounter reprises a Wimbledon semifinal from 2011 and a Roland Garros quarterfinal from this year, both of which tilted in the Serb’s direction after considerable ado.  In those two marquee matches, Tsonga electrified the audience and threatened Djokovic with his brand of exuberant, effortless shot-making, but he could not sustain leads large or small as substance trumped style in the usual trend at majors.  The best-of-three format at the Olympics awards a lesser advantage to substance, and Tsonga tellingly has accumulated a winning record against Djokovic under those circumstances.  When his game catches fire, he can hurtle past virtually anybody before they can catch the breath.  The Frenchman lacked much opportunity to catch his own breath, however, after a 48-game final set against Raonic that lasted three hours.  Despite defeating the inflammable Lopez a day later in an impressive display of endurance, he may find that the extended court time in singles and doubles depletes his fitness in another physical encounter.  Extended to final sets in two of his first three matches, Djokovic has carved a route through the draw that recalls his path through the early rounds of Roland Garros:  precarious, tumultuous, but ultimately  bulletproof when matches hung in the balance.  While the underdog should gain confidence from those tremors of unease emitted by the favorite, he has not defeated him since early 2010, when an entirely different level of competitor confronted him.  One could argue that the Frenchman also has improved dramatically since 2010, so spectators might witness a superb display of fast-court tennis with strength pitted against strength in Tsonga’s serve against Djokovic’s return.


Azarenka vs. Kerber:  When they met at an Indian Wells semifinal, the world #1 experienced minimal difficulty in outmaneuvering a German still adjusting to her newfound stature.  Neither able to outhit Azarenka from the baseline nor outlast her in extended rallies, Kerber appeared tactically uncertain and frustrated by her opponent’s sparkling returns.  A finalist at Eastbourne and semifinalist at Wimbledon, Kerber covers even fast courts with an alacrity deceptive considering her robust physique.  She displayed a mature composure in winning two tiebreaks from Venus in the previous round, a situation in which a challenger of lesser steel might have bowed to the reputation of the seven-time major champion or lacked the ruthlessness to end this legend’s final Olympics.  After a ghastly serving debacle in her opener, the world #1 has settled into less perilous form albeit less convincing than her march through five rounds at Wimbledon.  Nevertheless, Azarenka has not won a title since Indian Wells in March, reaching only one final during that span, so she does not intimidate as much as she did when Kerber last faced her.


Serena vs. Wozniacki:  Like Federer, one of the women seeking a career Golden Slam sets her sights on a rival who bears some resemblance to her nemesis in an Olympics quarterfinal four years ago.  Just as Dementieva halted Serena one match short of the medal rounds with suffocating baseline defense and penetrating groundstrokes, Wozniacki will attempt to do the same as long as she can survive the American’s serve, much more formidable on grass than on Beijing’s medium-speed hard courts.  But the Dane can gather confidence from having conquered the 14-time major champion at her home tournament in Miami, a stunning result that caused some to question the wisdom of the veteran’s comeback.  No such doubts linger now after Serena has won 31 of the 32 matches that followed, including four titles.  All the same, Wozniacki again won a set from a listless champion in Rome before the latter reared to life and served her off the court.  The Wimbledon 2006 junior champion never has reached a quarterfinal at the professional event, so she now ventures into uncharted territory at the All England Club.  Put simply, this match rests on Serena’s racket.  If she enters in one of her “foul moods” or inexplicably uninterested, she likely will lose.  If she stares down Wozniacki with the same intensity that flustered Jankovic and Zvonareva, she almost certainly will win. 


Sharapova vs. Clijsters:  Across the net from a four-time major champion who ardently covets a medal after a triumphant comeback stands—a four-time major champion who ardently covets a medal after a triumphant comeback.  Through three placid victories, Clijsters has not lost more than four games in a set and has held serve with relative ease outside a sporadic sprinkle of double faults.  Far more dramatic was the progress of Sharapova through an edgy encounter with home favorite Laura Robson and then a characteristically gritty, ruthless march back from a one-set deficit against Lisicki, the woman who toppled her on the same court earlier this summer.  Whereas the Belgian needed less than an hour to cruise through her Wednesday victory, the Russian slogged through nearly three hours of suspenseful tennis.  Sharapova has specialized in winning exactly those encounters over the last eighteen months, so Clijsters must beware if this quarterfinal travels into a third set as well.  Although she holds the ascendancy in their rivalry, the younger, taller blonde has won three of their last four battles and held match points in the fourth.  In an ironic twist that few would have expected five or six years ago, Sharapova has become the more consistent player lately both during matches and from one match to the next.  If each woman responds with courage to the magnitude of the moment, they should produce a compelling quarterfinal contrasting a shot-maker who has scorched 97 winners this week against one of the WTA’s most natural athletes and seamless movers.  What a pity that both cannot reach the medal matches.


Kvitova vs. Kirilenko:  Greeted with a relatively modest draw, Kvitova once again made her progress more difficult than necessary when she dropped sets in each of her first two matches.  She then elevated her efficiency in dispatching the unexpected Pennetta and should charge into the medal rounds by overpowering the lesser of the Russian Marias.  At their Australian Open meeting, Kirilenko failed to win a single game before waving the white flag in a retirement that some found dubious.  If she in fact sustained no injury other than to her pride, she can carry little hope into a collision with a Czech steamroller who allowed her just four games in the Fed Cup final last fall.  A year before Kvitova thundered into prominence by reaching a Wimbledon semifinal, Kirilenko ousted her from that tournament despite the vast disparity in their shot-making abilities.  Still rather raw, Petra chronically lacks discipline against players with more patience, if they survive the first few strokes of the rally.  As long as she does not confuse one Russian Maria with the other, however, she should connect with a sufficient number of vicious returns and well-placed lefty serves to keep Kirilenko at bay.  To paraphrase Franklin Roosevelt, the only thing that Kvitova has to fear here is Kvitova herself.