India's Sania Mirza thrashed fourth seed Nadia Petrova 6-2 6-1 at the Acura Classic to spring a surprise in San Diego on Wednesday.
The world number 59 had to win three qualifying matches just to get into the tournament, but made short work of her illustrious opponent.
"I knew that as soon as I would have relaxed, even for one second, she would have jumped on me," said Mirza.
Meanwhile Kim Clijsters beat Poland's Marta Domachowska 6-3 6-1.
Sixth seed Mary Pierce beat Anna-Lena Groenefeld 6-1 6-1 while Ai Sugiyama defeated 14th seed Daniela Hantuchova 7-5 4-6 6-2.
"I was really happy, especially with my first set," said Pierce. "I took Anna-Lena extremely seriously because I had never played her in singles before.
"I came out from the beginning, the first point of the match, ready to play. It was nice to see how well I was playing."
Fifth seed Patty Schnyder overcame Kveta Peschke 6-4 6-7 (2-7) 6-1 and American Mashona Washington secured a 6-4 4-6 6-4 victory over eighth-seed Vera Zvonareva.
Elena Likhovtseva, the ninth seed, beat Israel's Shahar Peer 0-6 6-2 6-4 while 10th seed Nathalie Dechy also came from a set down to record a 4-6 7-5 7-6 win over Gisela Dulko.