Madison Keys produced a comeback to beat Angelique Kerber at the Cincinnati Masters, while world number one Simona Halep rolled on.

Keys lost the opening set but hit 55 winners to upstage Wimbledon champion and fourth seed Kerber on Thursday.

Rain wreaked havoc throughout the day just like it did for Halep on Wednesday, but the top seed returned to complete her victory against Australian qualifier Ajla Tomljanovic.    

Two-time Wimbledon winner Petra Kvitova saw off Kristina Mladenovic, but US Open champion Sloane Stephens was sent packing by Elise Mertens.

KEYS RALLIES PAST KERBER

American 13th seed Keys survived to defeat Kerber 2-6 7-6 (7-3) 6-4.

Keys trailed 1-7 in her head-to-head record with Kerber but came from behind to book a quarter-final berth.

"Considering the last - what was it, six times? - it didn't go so hot, this victory feels good," Keys said. "More than anything, it was just how I handled being down and not letting the previous results kind of spiral in my head for me. I think that's what I'm happiest with."

Next up is in-form Aryna Sabalenka, who stunned sixth seed Caroline Garcia 6-4 3-6 7-5.

HALEP WAITING FOR BARTY CLASH

After overcoming Tomljanovic 4-6 6-3 6-3, French Open holder and Rogers Cup champion Halep set up a meeting with Ashleigh Barty.

But that clash was pushed back to Friday due to the rain in Cincinnati.

"It's a tough opponent. We played in Canada last week, so I expect a tough one. Hopefully, I will be at my highest level of tennis to win the match," Halep said of Barty.

STEPHENS SHOCKED BY MERTENS

World number three Stephens crashed to a 7-6 (10-8) 6-2 defeat against 15th seed Mertens.

Mertens – making her main-draw debut at the tournament – claimed five successive games in the second set as she went on to close out victory.

"It's really awesome to be able to play like this against a player like Sloane, who has a Grand Slam already," Mertens said afterward. "It gives you confidence."

Kvitova awaits Mertens after the eighth seed topped Mladenovic 6-4 6-2.

There were also wins for Lesia Tsurenko and Amanda Anisimova.