Victoria Azarenka and Elena Rybakina qualified for the semi-finals of the season-opening Brisbane International on Friday.
Azarenka battled for more than two hours before finally beating third-seeded Latvian Jelena Ostapenko 6-3, 3-6, 7-5 in the first match on Pat Rafter Arena.
The Brisbane tournament is a warm-up for the Australian Open, which the 34-year-old Azarenka won in 2012 and 2013.
Azarenka made a confident start as she bagged the first set with one break of serve, but as the match went deep Ostapenko made a comeback and levelled the match.
Ostapenko, the 2017 French Open winner, did some solid serving in the third set while Azarenka struggled to hold hers.
But at 5-6 in the third set, the Latvian faltered as a poor service game handed the Belarusian the win.
Azarenka will now face top seed Aryna Sabalenka or Daria Kasatkina in the last four.
"I thought the quality of tennis was incredible," Azarenka said. "She was blasting returns, blasting winners, so I had to stick in there.
"I was looking forward to seeing how my game was going to match up this year, and I think it's pretty good."
Meanwhile, Kazakhstan's Rybakina qualified for the semis when her Russian opponent Anastasia Potapova pulled out of the match with a stomach problem after losing the first set 6-1.
Rybakina quickly took a 4-1 lead when Potapova asked for the trainer. She played the next two games but withdrew when the 2022 Wimbledon champion Rybakina won the first set.
Rybakina will play Mirra Andreeva or Linda Noskova next.
In the men's draw, Russian Roman Safiullin beat Italy's Matteo Arnaldi 7-6 (7/4), 6-2 to set up a semi-final against either top-seeded Dane Holger Rune or Australian qualifier James Duckworth.
Spanish great Rafael Nadal plays his quarter-final against Jordan Thompson later Friday.
