NEW YORK -- In sheer terms of athleticism, Jo-Wilfried Tsonga has few peers. Toughness and, through an injury-filled career, fitness has sometimes been lacking.
But when the 26-year-old Frenchman sauntered into the fifth set against American Mardy Fish on Monday -- with a berth in the U.S. Open quarterfinals at stake -- he seemed confident. Maybe that was because he spent some time addressing his deficiencies in the Las Vegas hills before the summer hard-court season.
Those days with Gil Reyes, Andre Agassi's legendary trainer, paid off with a wind-blown 6-4, 6-7 (5), 3-6, 6-4, 6-2 victory. Tsonga had trailed two sets to one in a match that went 3 hours, 45 minutes.
"He said, 'Trust your legs, make it burn,'" Tsonga said. "I am really happy."
For Fish, it was the end of a terrific summer in which he won 17 of 21 matches and the U.S. Open Series crown. Fish, ranked No. 8 in the world, is having the best season of his career at 29 years old. His best effort in a Grand Slam event was reaching the quarterfinals at Wimbledon.
"I thought obviously this was my best [major] chance so far," Fish said. "I've made three quarterfinals and I've played [Rafael] Nadal, Nadal and [Andy] Roddick. Look, I had to go through Tsonga, [Roger] Federer,[Novak] Djokovic, Nadal, whoever's on the other side, just to win the tournament. That's a whole lot of guys."
Tsonga, for his part, has impressed, particularly at Wimbledon, where he beat Roger Federer in the quarterfinals before losing to Novak Djokovic in the semifinals. The key, he said, is having a clear head.
"My game is very good when I have nothing in my head, when I just play my game," Tsonga said. "When I don't think about other things like the wind, people in the box, all this stuff, the photographers. Sometimes it's tough for me, the [photographer's] click. Now I just relax and play my game."
"We practice really often, really, really often," Tipsarevic said of Djokovic. "Like in Montreal and Cincinnati, almost every second day. We know each other really well. We played doubles in Montreal together.
"In a way it can be easier on us maybe knowing where the other one is going to serve when it's important. But those things are two percent of the overall outcome of the match. Who plays better tennis is going to win. It's as simple as that."
Quiz time: Who is the only woman to reach the quarterfinals of three Grand Slam events?
That would be Germany's Andrea Petkovic, who dispatched Carla Suarez Navarro 6-1, 6-4. Afterward, one of the most astute players in the game offered a tutorial on confidence.
"First of all," Petkovic said, "I think after last year's breakthrough here when I reached fourth round, my confidence changed completely. I just started believing in myself I could reach the second weeks of Grand Slams. Before, I was just playing and seeing what happens. And now after that I really started believing in myself and I started focusing also and putting my plan as if I'm going to play for two weeks. That has changed.
"And definitely the experience of last year. I lost a lot of tough matches that I shouldn't have lost. I lost a lot of close matches. I crumbled under pressure. I crumbled on key moments. And this didn't happen to me that much anymore this year."
Time stands still: Monday's 16-14 tiebreaker, won by Djokovic over Alexandr Dolgopolov, was a marathon session, but it was two points shy of the amazing 17-15 breaker between Sam Stosur and Maria Kirilenko on Sunday night.
Kirilenko won the extra session -- the longest ever in a women's Grand Slam match -- but lost the match 6-2, 6-7 (15), 6-3. Three times in the tiebreaker Stosur appeared to have it won, but three Kirilenko replay challenges led to three overturns.
"I lost track of the score," Stosur said later. "Didn't know at one point if I was serving or receiving or when we should be changing ends, what was going on. Couldn't really hear myself think at times because it was so loud out there. Obviously it ended up being a record, so I've got another record here at the U.S. Open, which is cool."
Junior achievement: No. 1 junior boys seed Jiri Vesely of the Czech Republic began his journey through the draw with a 6-3, 6-1 victory over the Ukraine Vladyslav Manafov. Last year's boys' champion, Jack Sock, lost in the second round of the main draw to fellow American Andy Roddick. Madison Keys, the 16-year-old American who won her opening-round match in the main draw only to lose to Lucie Safarova in the second, struggled in her opening junior match. The No. 12 seed prevailed 6-1, 6-7 (1), 6-3 over Emily Fanning of New Zealand.
Greg Garber is a senior writer for ESPN.com.