Fifth seed Jo-Wilfried Tsonga suffered a second-round upset Thursday at the 2012 U.S. Open.

The former Australian Open runner-up was shown the door by Martin Klizan, 6-4, 1-6, 6-1, 6-3, at Armstrong Stadium. The 52nd-ranked Slovak pulled off the stunner in 2 hours, 12 minutes by breaking a sluggish Tsonga a whopping seven times on Day 4.

Tsonga is the second top-10 seed to fall this week, joining No. 10 Juan Monaco.

"Sometimes I'm in good shape, sometimes I'm not," Tsonga said at his post- match press conference.

The powerful Tsonga was a U.S. Open quarterfinalist last year and reached at least the quarters at this year's French Open and Wimbledon events, including a trip into the semis at the All England Club in July.

In other action involving top-20 seeds, sweet-swinging No. 11 Nicolas Almagro of Spain outlasted talented German Philipp Petzschner 6-3, 5-7, 5-7, 6-4, 6-4, No. 12 Croat Marin Cilic snuck past German Daniel Brands, 6-3, 6-2, 5-7, 4-6, 7-5, and No. 17 Japanese Kei Nishikori spanked American Tim Smyczek 6-2, 6-2, 6-4 on the hardcourts at the USTA Billie Jean King National Tennis Center.

American Mardy Fish, seeded 23rd at the fortnight, started out slowly against former top-five Russian Nikolay Davydenko before overcoming a two-sets-to-love deficit en route to a 4-6, 6-7 (4-7), 6-2, 6-1, 6-2 victory at Ashe Stadium. Davydenko is a two-time U.S. Open semifinalist.

Fish needed 3 hours, 27 minutes to complete his comeback, which he did with the help of 16 aces and 10 double faults by Davydenko, who was broken eight times on Day 4.

Meanwhile, 30th-seeded left-hander Feliciano Lopez outlasted fellow Spaniard Pablo Andujar 6-4, 6-1, 6-7 (5-7), 3-6, 7-5 and promising 19-year-old American Jack Sock whipped Italian Flavio Cipolla 6-2, 6-2, 6-4. Lopez is rewarded with a match against Olympic gold medalist, Wimbledon runner-up and former U.S. Open finalist Andy Murray, while Sock will meet Almagro in the round of 32.

Later Thursday, world No. 1 Roger Federer will play his second-rounder against speedy German Bjorn Phau. The 17-time Grand Slam king is the reigning Wimbledon champion and is a five-time U.S. Open titlist.