Brisbane - Naomi Osaka battled for two hours while Petra Kvitova cruised to victory on Friday as both booked spots in the semi-finals of the season-opening Brisbane International.
Osaka's form slipped after the first set but the world No 4 eventually saw off sixth seed Kiki Bertens 6-3, 3-6, 6-3 at the Pat Rafter Arena.
Bertens was on the attack in the second set but lost control in the decider when she began to struggle with her first serve.
It was a more straightforward affair for fifth seed Kvitova, who downed American qualifier Jennifer Brady 6-4, 6-2.
Kvitova, a two-time Wimbledon champion who won her first WTA title in Brisbane in 2011, was in superb touch against Brady, who beat Maria Sharapova and Ashleigh Barty in the first two rounds but was never in the hunt.
"I didn't serve well at all the first game, but since then I served much, much better and that's helped me a lot through this match today for sure," Kvitova said.
"My body's still not falling apart yet, so that's good as well."
Osaka, who burst to stardom by winning consecutive Grand Slam crowns at the 2018 US Open and 2019 Australia Open, will next play defending champion Karolina Pliskova.
Pliskova reached the semi-finals with a tough 7-6 (8/6), 6-3 over unseeded American Alison Riske.
The world No 2 never had it easy against Riske, who let herself down with 11 double faults.
"It's always close - I don't know what it is about me and Alison," Pliskova said.
"I think we played really good tennis today. We actually have very similar games. It's always close and always seems to have tiebreaks.
"I'm happy to win in two sets."
Kvitova faces Madison Keys, who cruised past fellow American Danielle Collins 6-4, 6-1 to secure her berth in the final four.
Collins seemed flat after two impressive victories in the opening rounds and her opponent never ceded control of the match.
"I don't think Danielle played her best tennis today, but I think I did a really good job taking care of my side of the net," Keys said.
"I served well, returned fairly well, especially in the second set. So I think overall it was just a pretty solid match."
Results from matches played on the fifth day of the Brisbane International at the Queensland Tennis Centre on Friday (x denotes seed):
Madison Keys (USA x8) bt Danielle Collins (USA) 6-4, 6-1
Petra Kvitova (CZE x5) bt Jennifer Brady (USA) 6-4, 6-2
Naomi Osaka (JPN x3) bt Kiki Bertens (NED x6) 6-3, 3-6, 6-3
Karolina Pliskova (CZE x2) bt Alison Riske (USA) 7-6 (8/6), 6-3