CINCINNATI, OH, USA - No.8 seed Petra Kvitova became the first player through to the semifinals at the Western & Southern Open thanks to a protracted 7-5, 5-7, 6-3 win over No.15 seed Elise Mertens in Friday's quarterfinal. 

"Luckily I did have good second serves, which was very important for me to put it back to the court and play some rallies," Kvitova told the press, after her win. "I really need to play more aggressive. That's how I made those important points."

The two-time Wimbledon champion advances to a semifinal on U.S. soil for the first time in nearly two years, having last done so at the Connecticut Open in 2016, and is back in the last four in Cincinnati for the first time since 2012.

Until this year’s run, Kvitova had failed to win back-to-back matches on any other visit to the Lindner Family Tennis Center, having reached the round of 16 after first round byes in 2011 and 2013.

The World No.6 held off a spirited charge from the Belgian in the opening set, but dropped a topsy-turvy second set and needed to come back from a break down in the final set to complete a victory in the pair's first-ever meeting after a grueling two hours and 42 minutes of play. 

"Definitely she's a tough opponent," Kvitova said, regarding Mertens. "And when I gave her time and I didn't really play that aggressive, she was just going for that. I was the second player on the court, which I didn't want."

The high-risk affair was marked by a large amount of winners and unforced errors, but Kvitova moved through with 43 winners to Mertens' 19, and the Czech won two-thirds of points on Mertens' second service.

Kvitova moves on to play Kiki Bertens in the semifinals, after Bertens upset Elina Svitolina later on Friday. "She improved a lot," Kvitova said of Bertens. "Overall, she's serving big, and of course when we are serving better, then we are making a little bit easier points and it's just tough to break."

The opening set was one of runs, and it began squarely in favor of the two-time Wimbledon champion. Kvitova streaked through the first five games, setting the tone as she won the first 10 points of the match overall, and was two points away from winning the opener on three occasions. 

Mertens hung tough as she worked her way into the match, however, as she responded with a five-game run of her own, and even held two break points in a pivotal 11th game which would've have given her her first lead of the match had she converted.

Nonetheless, Kvitova got herself over the line on her third set point, all of which came in Mertens' final service game of the set, with a blistering backhand return that was too hot for the Belgian to handle. 

The second set proved the inverse of the first, as it was Mertens who streaked through a run of games of establish an early edge.

The Belgian No.1 broke to love to begin the set, and held on to a lengthy, five-deuce service game to consolidate her lead. 

Another love break followed, as Mertens won the first nine points played on the Czech's lefty serve in the set, and the World No.14 consolidated a 4-0 lead. 

From 0-15 behind on her serve in the fifth game, Kvitova ran off a run of eight straight points to her side of the scoreboard, and pocketed a third straight game behind a hold from 0-30 down. 

The Czech's momentum continued into the ensuing games, as she broke the Belgian to love to level the set at 5-5.

Nonetheless, Mertens bounced back in style, as she denied Kvitova two game points for a 6-5 lead, and converted on her second break point of the game to restore her break advantage. The Belgian consolidated with a quick love hold to send the match into a deciding set.

Mertens continued her momentum as the deciding set began, opening with a break of the Kvitova serve and using every ounce of energy she had to run down all of the Czech's shots, en route to a 2-0 lead.

But the Czech lefty finally started to click on her vaunted forehand again, getting back on serve at 2-2 after an up-and-down game, and breaking once more for 4-2 after forcing an error from the Belgian with her biggest weapon.

Kvitova double faulted while down break point in the next game, ending her four-game win streak and allowing Mertens once last chance on serve. But the No.8 seed took the Belgian's serve one final time, breaking on her third chance in the following game via a strong service return.

At 5-3, Kvitova refused to let Mertens back into the match, and closed out her victory on serve to become to first player into the final four in Cincinnati.