No.6 seed Karolina Pliskova regained the upper hand in her rivalry with Jelena Ostapenko at the Porsche Tennis Grand Prix, winning 6-7(7), 6-4, 6-3 in two hours and 23 minutes to reach her second quarterfinal of the year.

The 21 aces Pliskova fired tied her career-best amount in a single match, having previously struck the same number in beating Dominika Cibulkova 6-4, 4-6, 6-3 in the 2017 Doha semifinals. This is the fourth time that the Czech has landed more than 20 aces in a match.

Having needed three sets to get past lucky loser Tamara Korpatsch in the first round, Pliskova was satisfied with her improvement.

"I did many things today much better than I did yesterday," she said. "I'm proud of myself that I'm able to change that performance in a day. I felt I played really good tennis today - also, she played a really good match. We know that we can really have a lot of unforced errors, which was not the case today.

"For some reason she likes to play me, I think, so she’s always playing really good tennis against me. But it was a great level and I thought I was serving extremely well today for all the match - all the three sets, even though the first set was a bit unlucky."

The pair's head-to-head had been deadlocked at three apiece ahead of this clash, with Ostapenko having won their past two meetings. The Latvian's 6-4, 6-2 dismissal of Pliskova in the second round of Roland Garros had been one of the most emphatic wins in the series. Thursday's result, though, was a throwback to another epic in the same venue - Pliskova's 5-7, 7-5, 6-4 victory in the 2018 Stuttgart quarterfinals en route to the title.

Three years on, Ostapenko again had the upper hand in the first half of the match. From 2-4 down in the first set, the World No.52 overpowered Pliskova with greater intensity off the ground, and rose to the occasion on almost every big point.

Pairing aggression with accuracy, Ostapenko fended off three break points at 5-5 and saved a set point in the ensuing tiebreak. Her ability to change the direction of the ball repeatedly left her opponent flat-footed, and the frustrated Pliskova smashed her racquet between the first and second sets.

"Actually I thought it helped me. I played even more aggressive and I just went for my shots."

- Karolina Pliskova on bouncing back after breaking a racquet.

"It was really frustrating, but I also thought I was playing quite good tennis," said Pliskova. "So I didn’t really want to give up only because I lost the first set. Also, she is a great player. She has some good shots which I have to accept. So, I released a bit of my tension which was in me after that set. I broke that racquet - but actually I thought it helped me. I played even more aggressive and I just went for my shots."

Midway through the second set, Pliskova indeed raised her game. Stepping into court to punish Ostapenko's second serve, the 29-year-old found three clean return winners en route to breaking for 4-2.

Ostapenko had her chances to level the set, saving set points at both 2-5 and 3-5, and breaking Pliskova back with a return winner of her own. But the 2017 Roland Garros champion's high-risk strategy was now less reliable, and an eighth double fault conceded the set.

In the decider, Pliskova's serve rose to new heights. The former World No.1 hammered down 10 aces in the third set alone - at least one in every service game. Having captured the crucial break for 4-2 after an Ostapenko backhand sailed long, Pliskova maintained that lead to get over the finishing line.

A pair of double faults serving for the match were just a minor blip, and Pliskova sealed her first match point after another wayward Ostapenko forehand.

The last time Pliskova defeated Ostapenko in three sets in Stuttgart, she went on to lift the trophy. Having posted consecutive match wins for only the second time in 2021, she will be seeking to repeat that run, with a quarterfinal against No.1 seed Ashleigh Barty the next challenge.

Pliskova was joined in the last eight by No.5 seed Aryna Sabalenka, who played a superb match to defeat qualifier Anna-Lena Friedsam 6-4, 6-2 in one hour and 19 minutes. The Belarusian, who is making her tournament debut in Stuttgart, struck 34 winners to 20 unforced errors and converted all four of her break point opportunities. A series of delightful dropshots also demonstrated her confidence in an ever-increasing on-court repertoire.

World No.125 Friedsam also contributed her own highlights to a high-quality contest, posting a ratio of 24 winners to 11 unforced errors, but was able to take just one of her seven break point chances.