Top seed Serena Williams edged Australian Samantha Stosur in two gripping tie-breaks to advance to the third round of the Western and Southern Open in Cincinnati.
Stosur beat the American in the 2011 US Open final, but Williams has since gone on to win their last three encounters.
Neither player could force a service break in either set, but Williams made the decisive mini-break in both tie-breaks to seal a 7-6 (9/7) 7-6 (9/7) in an hour and 56 minutes.
Stosur led both breakers at one stage, but Williams was able to rally from 4-0 down in the first and 6-4 down in the second.
Third seed Petra Kvitova was knocked out by rising star Elina Svitolina.
After surprisingly dominating the first set, Svitolina was given a much tougher challenge in the second, but the recent Baku champion held firm to close out a 6-2 7-6 (7/2) victory against the Wimbledon champion.
"I'm disappointed with how I played today," Kvitova told wtatennis.com. "From the beginning, I didn't feel very comfortable. I really tried to fight, but it just didn't happen for me.
"I'm not sure why. I'm glad that I entered New Haven, so I still have a chance to get more matches under my belt before the US Open."
Polish fourth seed Agnieszka Radwanska, who won the title in Montreal last week, safely saw off Japan's Kurumi Nara 6-2 6-2.
Anastasia Pavlyuchenkova will take on Maria Sharapova next after beating Karin Knapp 6-1 6-3 while Lucie Safarova beat Zarina Diyas 6-4 6-2 to set up a clash with Simona Halep.
Sabine Lisicki needed three sets to overcome 14th-seed Sara Errani 6-4 2-6 7-6 (7/2) while Sloane Stephens beat Barbora Zahlavova Strycova 7-5 6-1.
Sixth-seeded Angelique Kerber was a 6-4 6-1 victor over Ekaterina Makarova and Carla Suarez Navarro prevailed 6-3 7-5 over Pauline Parmentier.
Ninth seed Ana Ivanovic was too strong for Christina McHale as she raced to a 6-4 6-0 victory, while fellow Serb Jelena Jankovic, the No 8 seed, ousted Annika Beck 6-1 7-6.