Daniela Hantuchova ousted defending champ Aleksandra Wozniak 6-4, 4-6, 7-5 in a battle lasting 2 hours and 36 minutes.
Serena Williams, the top seed in the tourney, beat Li Na in the opening set 6-3, but ran into stubborn resistance from Li, who saved four match points in the second set and went to a tie-breaker before finally succumbing 7-6.
"I was playing a tough player who never gave up," said Williams, who admitted she had been overly eager to put the match away. "She's always playing so well and it's important not to get anxious."
Serena's sister, Venus, the No. 2 seed, has less difficulty in defeating Stephanie Dubois 6-2, 6-3.
Other winners Tuesday included No.4 seed Jelena Jankovic and No.8 seed Marion Bartoli.