Serena Williams defeats Ana Ivanovic, 6-3, 6-4, to reach the U.S. Open quarterfinals. Williams has dropped the fewest games of any woman still in the field (10). (Al Bello/GETTY IMAGES)

As an athlete determined to wring the most out of the late stages of his career, Mardy Fish shed 30 pounds, toned his muscles, increased his stamina, swapped nights on the town for room service and made sleep a priority.

Fish controlled every variable he could these last two years to forge himself into a top-10 tennis player.

But Monday at the U.S. Open, the 29-year-old Fish ran headlong into variables beyond his control. Among them: Gusting winds, chatter from the crowd, an ailing right hip and the force that is Jo-Wilfried Tsonga.

And it was Tsonga who proved the fitter man, physically and mentally, shutting out everything but the next point to endure a 3-hour 45-minute battle of momentum shifts and advance to the quarterfinals, 6-4, 6-7 (7-5), 3-6, 6-4, 6-2.

“My game, you know, is very good when I have nothing in my head—when I just play my game, when I don’t think about other things like the wind, people [talking] in the box, all this stuff,” said Tsonga, 26, of France. He’ll next face five-time U.S. Open champion Roger Federer, who dispatched Juan Monaco with ruthless efficiency, 6-1, 6-2, 6-0, in an 82-minute match that didn’t start until 11:50 p.m. Monday.

“Obviously I’m disappointed that I didn’t go further here,” Fish said, adding that neither the wind nor his hip dictated the outcome. “I lost to a great player today. There’s just no way around it.”

Fish’s defeat leaves three American men in the hunt for a place among the tournament’s final eight. If weather permits, Andy Roddick, John Isner and wild-card Donald Young will vie for a quarterfinal berth Tuesday at the USTA Billie Jean King Tennis Center.

Serena Williams succeeded Monday in reaching the quarterfinals after missing nearly a year of competition because of injury and illness. A three-time U.S. Open champion, Williams easily handled the 20-mph gusts to dispatch former world No.1 Ana Ivanovic, 6-3, 6-4.

She moves on to face 17th-seeded Anastasia Pavlyuchenkova, who upset No. 7 seed Francesca Schiavone, 5-7, 6-3, 6-4 earlier in the day.

Also advancing was top seed Novak Djokovic, who survived a thrilling first-set tiebreak to defeat a free-hitting youngster from Ukraine, Alexandr Dolgopolov, 7-6 (16-14), 6-4, 6-2.

Djokovic, a two-time U.S. Open finalist, extended his 2011 record to 60-2 with the victory and earned a quarterfinal meeting with his good friend, fellow Serbian and Davis Cup teammate Janko Tipsarevic, who defeated Juan Carlos Ferrero of Spain, 7-5, 6-7 (7-3), 7-5, 6-2 earlier in the day.

And in a test of fitness and focus, Caroline Wozniacki proved why she’s the world’s top-ranked player and tournament’s No. 1 seed, persevering after losing the first set and trailing 1-4 in the second to defeat 2004 U.S. Open champion Svetlana Kuznetsova, 6-7 (8-6), 7-5, 6-1.

The match lasted more than three hours. And the toll was evident on Kuznetsova, who committed 78 errors, while Wozniacki rarely changed expression or breathed heavily. She’ll face 10th-seeded Andrea Petkovic in the quarterfinals.

Fish was the United States’ highest seed (eighth) and best hope of claiming the men’s title. And he had sailed into the fourth round without losing a set, as had the 26-year-old Tsonga.

Despite their combined 20 years on the pro tour, Fish and Tsonga had never met on the court. So the first set at Louis Armstrong Stadium was a race to see who could figure out his opponent and the wind more quickly.

Tsonga, an imposing 6 feet 2 and 200 pounds, won the opening set, capitalizing on two double-faults by Fish in the ninth game.

But the American closed the next set with a huge serve and forehand, then rolled to a two-sets-to-one lead.

With pain in his right hip, Fish called for a trainer before the fifth set. When play resumed, Tsonga seized the upper hand, winning the most thrilling point of the match — a furious exchange of reflex volleys — when Fish badly over-hit a passing shot to the open court.

Fish also complained about the decibel level of a Tsonga partisan in Tsonga’s guest box and uttered an expletive, presumably at the chair official, in the conversation that followed.

By then, Tsonga’s big forehand and deep reservoir of energy were simply too much for Fish to overcome.

With the wind so unpredictable, no one’s best tennis was on display Monday. Conditions called for prudence rather than daring.

Ivanovic picked Williams to win the tournament well before Monday’s match given her impressive form. Williams also has 13 Grand Slam titles — 13 more than any woman left in the field.

Asked if she felt she were the world’s best player despite her 28th ranking, Williams said: “I don’t know if I’m the best or not. I believe I am. And I think a lot of other girls, women in the locker room, believe they are too — as they should. I don’t think anyone should go out and say that they’re not. If you’re talking about self-belief, yeah.”