Russia's Daria Kasatkina pulled off a shocking upset of world No. 1 Angelique Kerber of Germany 7-6(5), 6-2, on Tuesday to advance to the semifinals of the Apia International Sydney tournament.
Kasatkina, a 19-year-old who is ranked 26th in the WTA singles rankings, entered the match with a 2-10 record against Top 10 opponents. She came within a point of upsetting world No. 7 Garbine Muguruza of Spain at the Brisbane International.
The top-seeded Kerber played from behind for most of the first set before saving a set point in the 10th game to force a tie-break, which the third-seeded Kasatkina claimed. Kerber battled back to 2-2 after Kasatkina broke serve in the second set.
Kasatkina broke serve twice more to pull out a win in the 93rd minute.
"This match was very important for me," Kasatkina said. "I was three matches from match point against Top 10 players, and today I beat No. 1 in the world. So I think I am somewhere close (to Angelique's level)."
Kerber, who is 12-3 against Top 10 competition, admitted she felt out of sync.
"The first matches of the year are always tough, and I'm still not feeling my rhythm," Kerber said. "I made too many mistakes. I was not feeling the ball.
"It's tough, but I think it was still good to have a match before going to Melbourne."
Kasatkina beat fourth seed Timea Babos of Hungary 3-6, 6-4, 6-2 in the opening round to move on to face Kerber.
Kasatkina will face sixth-seed and world No. 10 Johanna Konta of England in the semifinals. Konta beat sixth-seed Daria Gavrilova 6-1, 6-3 on Tuesday to advance.
"She has a good serve, good backhand. She doesn't have so many weakness," Kasatkina said of Konta. "It will be good match."
Second-seeded Agnieszka Radwanska of Poland rolled to a 7-6 (1), 6-1 win over American Christina McHale.
"I was struggling with a couple of things tonight, but a very good tie-break from my side, very pleased with that," Radwanska said afterward. "Obviously it was much better in the second set, more comfortable and confidence. It was also a couple of tight games but in the end I was the one who was make the right shot. Very happy to make that match in two sets."
Radwanska will face qualifier Duan Ying-Ying of China in the quarterfinals. Ying-Ying was a 6-3, 6-2 winner over American Coco Vandeweghe.