This U.S. Open finally got its first shocker.
After three days of the top players not only winning but winning decisively, fifth-seeded Jo-Wilfried Tsonga was upset by Martin Klizan of Slovakia in the second round. The 52nd-ranked Klizan won 6-4, 1-6, 6-1, 6-3 on Thursday.
Before Tsonga's loss, top-five seeds on the men's and women's sides had played 14 matches -- and won all 14 in straight sets.
Tsonga was the runner-up at the 2008 Australian Open and a semifinalist at Wimbledon this year. He had reached at least the third round in 18 straight Grand Slam trips.
"Today I was not in a good shape," he said. "I didn't play good tennis. It seemed like I couldn't hit the ball enough hard to put my opponent out of position. I don't really know why it was like this today, but sometimes it's happen with me."
The 23-year-old Klizan, meanwhile, had failed to make it past the second round in three previous Grand Slam appearances. He had never defeated an opponent ranked better than No. 49.
"I had no pressure," Klizan said. "If I lose, then I lose. I lose with (a) good player. But I won and I'm very happy. It means for me more that I beat finally a guy from top 10."
Top-seeded Roger Federer used a second-round victory to work on his net game, winning 32 points from the front of the court in a 6-2, 6-3, 6-2 victory over Bjorn Phau of Germany.
Federer also finished with 15 aces in a 90-minute victory over his 83rd-ranked opponent.
"I like coming forward when I am up," Federer said. "Just try maintaining that. And if things don't work out that way, you can always play it safe again."
Federer improved his U.S. Open record to 63-7 and stayed in the mix for his sixth U.S. Open title, which would be a record in the Open era.
Federer's third-round match is against No. 25 Fernando Verdasco.
This year's Open has generated plenty of drama in one area: comebacks from two sets down. American Mardy Fish rallied to beat Nikolay Davydenko 4-6, 6-7 (4), 6-2, 6-1, 6-2, the 10th time in this tournament a man has won after losing the first two sets - already an Open record.
The 30-year-old Fish, seeded 23rd, missed two months this season because of an accelerated heartbeat but showed few signs of fatigue in playing nearly 3 1/2 hours.
Jack Sock also reached the third round at a major tournament for the first time. The 19-year-old from Nebraska beat Flavio Cipolla 6-2, 6-2, 6-4.
Sock, ranked 248th, got in with a wild card. He saved 12 of the 13 break points he faced, while converting all six he earned on the 88th-ranked Cipolla's serve.
Next up is a meeting with No. 11 Nicolas Almagro of Spain, a 6-3, 5-7, 5-7, 6-4, 6-4 winner over Philipp Petzschner.
James Blake reached the third round at a Grand Slam event for the first time since 2010 with a dominant 6-1, 6-4, 6-2 victory over 24th-seeded Marcel Granollers.
Blake needed a wild card to get into the Open after his ranking fell outside the top 100.
Kei Nishikori, seeded 17th on the men's side, eliminated American Tim Smyczek.