Anastasia Myskina became the biggest casualty of the women's singles so far, losing to qualifier Russian qualifier Anna Chakvetadze in the second round.
The French Open champion was seeded fourth and had crushed Ludmila Cervanova 6-1 6-0 in the first round.
But she made 37 unforced errors against her compatriot and lost the first set 7-3 in a tiebreak.
Chakvetadze then won three out of the last four games to secure a 7-6 (7-3) 6-3 victory.
Myskina blamed tiredness for her defeat, which came after she lost out on an Olympic bronze medal at the Athens Games.
"It was a pretty bad match for me," said the fourth seed.
"I had nothing today, I didn't want to run, I didn't want to fight, I didn't want to do anything.
"I guess maybe the Olympics took a lot of my energy. When I got to New York I guess I was still crying from what happened in Athens."