The world number six, who lost in four sets to the Slovakian in New York, admitted that he struggled to cause problems for his opponent.
Tsonga said: "I was not in good shape. I didn't play good tennis. It seemed like I couldn't hit the ball hard enough to put my opponent out of position. I don't really know why it was like this, but sometimes it happens with me.
"I'm disappointed but it's tennis. Sometimes you win, sometimes you lose. Of course, it's always sad when you lose but I will wait for another good moment. I will forget it."
Klizan has claimed that the victory was the "best result" in his career.