No. 2-ranked Iga Swiatek lost to a player younger than 18 for the first time on tour when she was stunned by Mirra Andreeva in the Dubai Championships quarterfinals Thursday.

Andreeva, 17, won 6-3, 6-3 to become the youngest semifinalist in Dubai’s 24-year history and reach the first WTA 1000 semi of her career.

Swiatek won their only previous match in August in Cincinnati, losing the first set and winning 7-5 in the third. She conceded the first again Thursday and was a up a break in the second, but Andreeva reeled off the last five games, eventually sealing the victory when Swiatek’s return on match point went wide.

“I was nervous before the match,” said Andreeva, ranked 14th. “I told myself I played great in our last match and I just need to keep playing aggressive.”

It’s her fifth win over a top-10 player and second over a No. 2; she beat Aryna Sabalenka in the French Open quarterfinals last year.

Andreeva found herself on the ropes early as she struggled to hold serve in her first service game of the opening set but eventually held on to make it 1-1.

She then broke Swiatek and continued to impress, covering the court well while also reading her opponent’s shots to set up winners. Andreeva broke Swiatek once more, firing a forehand return down the line on set point.

The Russian teen will next face sixth-seeded Elena Rybakina or Sofia Kenin.

Clara Tauson, who knocked out Sabalenka on Wednesday, defeated Linda Noskova of the Czech Republic 7-6 (4), 6-4 to reach her third semifinals of the year. She will play Sorana Cirstea or Karolina Muchova.

The Associated Press and Reuters contributed to this report.