India's Sania Mirza defeated 15th seed Katarina Srebotnik of Slovenia 6-3, 7-6 (7-4) in the second round of the Acura Classic WTA Tour event in San Diego, California.
Sania, whose ranking has slipped to 44, scored the day's only upset as she defeated Srebotnik. The 19-year-old had defeated former world ranked No 11 Meghan Shaughnessy 6-3 6-1 in the first round.
In other matches, defending champion Mary Pierce struggled for more than two and a half hours before finally squeezing to a 6-1, 6-7 (7-9), 7-6 (7-4) opening victory over Sybille Bammer.
Sixth-seeded Pierce was playing for the first time in nearly six months after suffering a right foot ligament injury in February. Her last match was the Paris indoor final, which she lost to Amelie Mauresmo.
The 31-year-old Pierce survived a tougher-than-expected second-round test in coastal heat at the golf resort venue for the hard court tournament.
Russian fourth seed Elena Dementieva got on and off court with efficiency in a 6-2, 6-2 thrashing of American Ashley Harkleroad while Slovak 11th seed Daniela Hantuchova beat Olga Savchuk of Ukraine 6-2, 6-4.
Two Serbs won through, with 14th seed Ana Ivanovic putting out 33-year-old American Amy Frazier 6-3, 6-2, and Jelena Jankovic stopping Alana Bondarenko of Ukraine 6-1, 7-6 (7-4).
Russia's Vera Zvonareva advanced when Chinese Shuai Peng was unable to continue with heat exhaustion, 3-6, 7-6 (7-5).
"I could see she was having problems with the heat," said Zvonareva, winner of two titles since June including Cincinnati last month. "I felt fine and wasn't' thinking about the weather.
"I couldn't tell any difference in her game, since she goes for all of her shots and plays aggressive on every ball."
Zvonareva plays in the third round on Thursday against second-seeded Russian compatriot Maria Sharapova, with the pair level at two wins each.
But their last meeting came two years ago in the Los Angeles WTA Championships, where Sharapova won in round robin play.