TOKYO, Japan - No.4 seed Karolina Pliskova held off American Alison Riske in a marathon first quarterfinal at the Toray Pan Pacific Open on Friday, 6-1, 6-7(5), 7-6(4).

After being pushed to a final set by the American qualifier, Pliskova needed to come from a break down twice in the third set and saved two match points in a key 12th game before forcing a tiebreak and prevailing in over two-and-a-half hours.

The Czech last reached a semifinal at the Mutua Madrid Open in May, where she fell to compatriot and eventual champion Petra Kvitova.

That was the last of three semifinals the former World No.1 reached in the first half of the season, and it came the week after she claimed her lone title of the year to date at the Porsche Tennis Grand Prix in Stuttgart.

"It was a very important win for me. I think she was a little bit better, but I just managed to fight somehow and get the victory," Pliskova said on-court after the match. "It was very close... and I'm just happy that I'm the winner."

The American never settled in an opening set which lasted just about 30 minutes, undone in part by 15 unforced errors in seven games. After breaking Pliskova's serve to begin the match, Riske would not win another game as she fell behind, though three of those games extended well past one deuce. 

In the second set, however, Riske began to show some of the form that took her to a stunning upset of two-time Grand Slam champion Garbiñe Muguruza in the second round, routinely finding the corners with winners down the line.

After seven straight holds, it was the WTA World No.75 who broke first, with a sweetly struck forehand pass earning her a third break point in the set, and the first which she converted.

Nonetheless, a pair of backhand winners helped Pliskova get the set back on serve with a break to love as Riske sought to force a decider, and the two players exchanged holds until the set wound through to a tiebreak. 

Each player struck 18 winners over the course of the second set, with little to separate the two off the ground and at the service line as well.

In the final set, Pliskova was the first player broken in the fifth game, and later broke the American once more in the 12th game from 40-15 down to stay alive and force a final tiebreak.

"I felt maybe I'm going to have a chance if I played a good return, and I did it two times," Pliskova said of the match points.

"After that, she double-faulted because it was a hard rally for her. I tried to stay calm, fight and find my best game.

"I'm just happy that I was able to fight back. I was quite lucky a few times today... but that's tennis and I'm just happy to be in the semifinals here."

The Czech had never been past the quarterfinals in four previous trips to the Toray Pan Pacific Open when it was held at the Ariake Coliseum, having twice lost in the last eight in 2015 and 2017.

Pliskova struck 40 winners and 10 aces over the course of the victory, and converted six of eight break points that she held in the match, ensuring that she'd have a career-best result at the event's new home at the Arena Tachikawa Tachihi.

"I love Tokyo so much and also Japan, but I was never playing my best tennis in the old stadium," Pliskova said.

"I think it's lucky that they moved it to this stadium. I feel much better here. The support is amazing here and I'm enjoying every match, and hopefully, I can make it to the final."

Up next, Pliskova will face Croatia's Donna Vekic for a place in the final - a match she says "will be exciting".