Victoria Azarenka booked her place in the quarter-finals of the WTA Western & Southern Open after a topsy-turvy 6-3 6-4 victory over Magdalena Rybarikova.
The scoreline made it look comfortable enough for the second seed but the two players combined for 13 service breaks, including the last seven games of the contest.
"Today there was sometimes a little bit of trouble serving but overall I felt better than I did in my first match," said Azarenka afterwards. "I was much more controlling the game. It was really up to me today, I felt.
"I was doing everything I wanted to do but I still feel there can be a lot more improvement."
The result means Azarenka is through to the last eight in Cincinnati for the first time in her career.
The two-time Australian Open champion was forced to withdraw from the Rogers Cup in Toronto last week with a lower back injury.
Meanwhile, Agnieszka Radwanska was in impressive form against Elena Vesnina, the fourth seed crushing her Russian opponent 6-0 6-2 in just 50 minutes.
Reigning champion Li Na reached the last eight with a 6-4 6-4 triumph over Angelique Kerber.
Roberta Vinci beat her fellow Italian and doubles partner Sara Errani 6-4 6-3 to set up a clash with Jelena Jankovic, who was given a tough workout by American Sloane Stephens on her way to a 3-6 7-5 7-5 win.