CINCINNATI, OH, USA - Former World No.1 Karolina Pliskova booked her spot in the Western & Southern Open quarterfinals, dispatching Swedish qualifier Rebecca Peterson, 7-5, 6-4.

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The 2016 champion - who defeated Angelique Kerber in a memorable final that prevented the German from becoming World No.1 - is yet to drop a set in Cincinnati, but had to battle through two tough sets on Thursday before emerging victorious after one hour and 42 minutes on Stadium 3.

"I saw only a little bit her playing somewhere and was practicing next to her, so I just know that the forehand is quite big. She's solid from the forehand," Pliskova said after pairing with twin sister Kristyna to upset No.3 seeds Barbora Krejcikova and Katerina Siniakova in a 10-point tie-break.

Pliskova has a chance to return to World No.1 for the first time since 2017 at this tournament, and had to beat Peterson to keep those hopes alive. Up against an unfamiliar opponent - who defeated Wimbledon semifinalist Barbora Strycova in qualifying and Roland Garros semifinalist Johanna Konta in the first round of the main draw - the three-time WTA Ace Leader nonetheless raced out to a 3-0 lead to start.

Peterson soon leveled things up, but Pliskova remained in front, saving four break points in a marathon 11th game to keep the pressure on the qualifier, who she broke to 15 in the next game to claim the opening set.

"I think she was playing good, and of course she improved during the match. The beginning was not her best, but I had some chances to go 4-0, didn't make it, and then was kind of tricky but for sure happy I played two sets because it could go three. I was down a break in the second. But I think I played solid."

The second set saw Peterson again rally from an early break, this time taking things a step forward as she broke the Czech powerhouse a second time to take a lead for the first time in the match. 

"I was quite confident that I'm going to win because my game was working. So at the important moments I felt like I played always good. The serve was solid, too. Not that many aces but a lot of first serves. I think it was good match."

Ultimately, Pliskova found a way to power through, winning the last three games to secure her spot in the last eight.

In all, she struck an impressive 36 winners to 29 unforced errors, seven aces, and won a solid 74% of points behind her first serve.

Standing between the former World No.1 and the semifinals is 2004 winner Svetlana Kuznetsova, who stunned 2017 champion Sloane Stephens, 6-1, 6-2, later in the day.

"She didn't have many matches, but I think maybe sometimes can be good so she's a little bit more fresh and for sure excited.

"You can see I think she's more enjoying it a little bit right now. She has win couple matches here and there, in Toronto, and I think here too. I think she's quite confident, so she's back maybe where she kind of was.

"I remember we played that year when I won [the tournament] on the same court, so of course good memories from that match. Always I felt like when I played her I played good tennis because she gives you rhythm. You know, she just plays super solid. Doesn't miss much.

"So good match always with her. You know, I lost couple times, too, so I have to be super-aggressive and for sure chance."