Reigning champion Nadia Petrova and 2002 winner Venus Williams both won in straight sets to reach the last eight of the Bausch & Lomb Championships.
Williams is playing only her third tournament this year
Top-seeded Petrova hit seven aces in beating Zheng Jie of China 6-2 6-3 in the third round in Florida.
Petrova will face Russian compatriot and fifth seed Dinara Safina, who beat Katarina Srebotnik of Slovenia 6-3 6-1.
Unseeded Williams eased past fellow American Meilen Tu 6-2 7-5 and now meets France's Tatiana Golovin.
The eighth-seeded Golovin knocked out 12th seed Samantha Stosur of Australia 6-2 6-1.
Defending champion Petrova closed out her match in style, opening the final game with three of her seven aces in the match.
She narrowly missed making it four in a row, but claimed victory when Zheng punched a backhand into the net.
"I was hoping to make it four straight in that game," she said. "My service is a weapon that I can win a lot of points with. When you ace somebody it just feels great."
Petrova defeated her next opponent, Safina, two weeks ago at the Sony Ericsson Open in Miami and she has no doubt she can repeat the result.
"She's a tough opponent but I've had two straight, two-set matches that I won and I'm very confident," said Petrova.
Second-seeded Jelena Jankovic and sixth seed Ana Ivanovic set up an all-Serb clash.
Jankovic was broken twice in defeating Alona Bondarenko of Ukraine 6-2 6-0, while Ivanovic dispatched Nathalie Dechy of France 6-1 6-1.
Third seed Daniela Hantuchova cruised past unseeded Slovakian compatriot Dominika Cibulkova 6-1 6-4 and will now play 13th seed Sybille Bammer, who beat Karolina Sprem 6-2 6-3.