Top seed Venus Williams was beaten 2-6 6-1 7-6 by unseeded Bulgarian Magdalena Maleeva at the Kremlin Cup today.
World number two Williams was playing her first match in Moscow since 1997 after getting a bye into the second round.
I didn't play for a few weeks and I'm really tired," the American said. "She played really well at the right points and I made a lot of mistakes. I wasn't my best playing against her. It was really difficult".
Williams fired 13 aces but also committed eight double faults and 45 unforced errors in a 98-minute match. Maleeva, who had beaten Williams in two of their four previous meetings, said she was confident going into the match.
"Statistics are not that important, but I knew I could beat her," said the 23rd-ranked Bulgarian.
Home favourite Anna Kournikova withdrew from the $2.2 million tournament with an ankle injury.