Amanda Anisimova pulled off a stellar comeback on Wednesday to get the better of Iga Swiatek 6-7 (3/7), 6-4, 6-2 and book her spot in the last four of the WTA Finals in Riyadh.

Making her tournament debut this week, the fourth-seeded Anisimova secured the runner-up spot in the Serena Williams Group behind Elena Rybakina.

Rybakina completed round-robin play with a perfect 3-0 mark, thanks to a 6-4, 6-4 success against Russian alternate Ekaterina Alexandrova earlier in the day.

Anisimova improved her three-set record this season to an impressive 15-3 by posting her 10th top-10 win of the year.

"It's so funny, my mom keeps telling me: 'You know you've won like so many three-set matches this year? You're so strong'. I was actually thinking about that. Against Iga today it was so, so tough, but honestly, I enjoyed it," said Anisimova, who reached the finals at Wimbledon and the US Open this year.

"I'm so excited (to be in the semi-finals), this is surreal, especially for my first time playing here," she added.

Swiatek and Anisimova split their two previous meetings, with the former triumphing in the Wimbledon final and the latter avenging that loss in the US Open quarter-finals.

On Wednesday at King Saud University Indoor Arena, the Pole saved all four break points she faced before she clinched the tiebreak to bag the opening set in 65 minutes.

The quality was incredibly high throughout and little separated the pair until Anisimova finally converted a break point in game 10 of the second set to draw level and force a decider.

The American made the first move in the final set, breaking in the fourth game on a Swiatek double-fault. Another break sealed the deal for Anisimova over the six-time Grand Slam champion in two hours and 36 minutes.

This is the first time in Swiatek's entire career that she has suffered two successive losses after winning the opening set.

The 24-year-old Pole was at a loss for words when trying to explain the reasons behind her defeat.

"I felt good mentally, physically, and tennis-wise also it was nice, looking at the conditions and everything; so, I don't really get why I couldn't go out of the group," said a disappointed Swiatek.

"Maybe I won too much in the last years and this is karma. It's really hard for me to say. It feels weird. It's not like I'm expecting (to win), but from my experiences, if I put so much intensity and grit and I cared that much, it usually paid off. So we'll see if I keep working if it's going to pay off or not." (AFP)