STUTTGART, Germany - No.5 seed Karolina Pliskova roared back from a set down to defeat reigning French Open champion Jelena Ostapenko, 5-7, 7-5, 6-4, to reach her first semifinal at the Porsche Tennis Grand Prix.

"I think it was more mental because we were waiting so long for our match. The match before was great so it was tough to play after that. She started very well so in the first set, I was just trying to hang in there and be a good opponent for her. I'm just lucky that I won in the end in three sets, but I'm happy I was able to hold my serve and not play a tie-break."

Ostapenko was aiming to move into the Top 4 with a win on Friday night. By Saturday morning it was the former World No.1 who had the better of the Latvian youngster after two hours and 18 minutes in a high-quality battle that saw both hit more winners than unforced errors.

Though Pliskova appeared out of sorts to start the match - quickly falling behind 0-3 - it was an even contest from there as the two traded body blows from the baseline. The tour's perennial ace leader served her way out of trouble in the second set when she was a game from defeat at 4-5 and won three straight games to level the contest.

Ostapenko never let up, striking 50 winners to 37 unforced errors, and had three break points for 5-5 as Pliskova served for the match, but it was the Czech's 14 aces that won the day, coming through in crucial moments to help put her over the finish line a little after midnight.

"It's tough with her because she goes full power into every return. You have to put a lot of first serves in and that's what I was trying to do in the last game. I was lucky a little but because she was up 15-40, playing more aggressively than me in the end. I'm happy my serve saved me because I hit a few aces. I'm so happy to be in the semifinals for the first time here."

It was nonetheless an impressive return to clay for the World No.5, who led Latvia to a Fed Cup victory over Russia last weekend before dropping just three games to Zarina Diyas in her opening round in Stuttgart.

As for Pliskova, the 2016 US Open finalist next faces Estonia's Anett Kontaveit, another marathon woman who kicked off Friday's night session with a hard-fought victory over Anastasia Pavlyuchenkova to also reach the Stuttgart semifinals for the first time.

"Normally I can sleep very well, but after such a match, I think it'll be pretty late. I'll try to be ready for my next match, which will be today, already! We'll see how it goes."