* Slovakian Klizan pulls upset in four sets
* Tsonga cannot explain poor performance
* Frenchman defends frequent play (Adds details, quotes)
Frenchman Jo-Wilfried Tsonga was at a loss to explain what went wrong after he suffered his earliest grand slam exit in five years at the U.S. Open on Thursday.
The fifth seed was beaten 6-4 1-6 6-1 6-3 by Slovakian Martin Klizan in the second round.
Tsonga said he was fully fit and healthy but that for some unknown reason he could not find his best form.
"I'm not very satisfied," Tsonga said. "I'm used to playing the round of 16 or quarter-finals, sometimes semis. ...
"My opponent was better than me the whole match I think, and that's it."
It appeared normal service had been restored when Tsonga won the second set at a canter but just as quickly, his game disappeared again.
World number 52 Klizan had never been beyond the second round at a grand slam event but the left-hander surprised Tsonga with some inspired tennis.
Tsonga rallied from 4-1 down in the fourth set to 4-3 but the 23-year-old Klizan held his nerve to seal the biggest victory of his career.
"I feel great," Klizan said. "I had no pressure. If I lose, then I lose to a good player. But I won and I'm very happy. It means for me more that I beat finally a guy from the top 10."
Tsonga had been a semi-finalist at Wimbledon last month and a quarter-finalist in New York last year.
"Today I was not in a good shape and I didn't play good tennis," he said. "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 happens with me."
He has been criticised for playing too much tennis - Janko Tipsarevic is the only top-10 player to have played more events in the past year - but Tsonga said he had little choice.
"If you're (ranked) four or five, to be at this ranking I have to be competitive with the other guys who play (every) week," he said.
"I'm not a machine. Sometimes I'm tired, sometimes not. Sometimes I am in good shape, sometimes not."