Top seed Svetlana Kuznetsova had a struggle to overcome Czech Klara Koukalova 7-5 6-3 in the second round of the San Diego Classic on Tuesday.
It was not such a good day for Kuznetsova's fellow Russian Elena Dementieva, the third seed going out 7-5 6-4 to China's Shuai Peng.
Playing her first match in more than a month, U.S. Open champion Kuznetsova looked rusty and was unable to impose her normal authority on the match.
"It was tough, I wasn't playing my tennis," said Kuznetsova. "I felt like I was playing defensive tennis against her best offensive tennis."
Kuznetsova, who trains in Spain, said she had been practising on a slower hard court with different balls for the last couple of weeks and was not used to the conditions yet.
"It was like she was hitting every ball at my knees," she said. "I couldn't find a rhythm. But that's what top players do - they win matches even when they're not at their best."
Playing only her second World Tennis Association (WTA) Tour match since almost upsetting Lindsay Davenport at the French Open, China's highest-ranked player Peng outran Dementieva, staying away from the Russian's forehand and dominating off the backhand side.
"I wasn't thinking I was playing a top player, only to play my game and fight for every point," Peng said.
"I'm happy that I beat a higher ranked player, but I want to keep improving," reports Reuters.