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World number two Swiatek made 36 unforced errors in the last two sets while Rybakina made 17, with the Kazakh player getting her first win over the six-time Grand Slam champion in their last five meetings.

Rybakina, who beat Anisimova on Sunday, reached the semi-finals after her second straight win.

"It was difficult to be down, but in the second set I pushed myself, the serve improved. Really happy that I stepped in and played better in each point," Rybakina said.

Swiatek made a dominant start to win the first three games as she targeted Rybakina's body with her serves, while the sixth seed's backhand returns often hit the net. Swiatek looked in control as she took the first set 6-3.

However, the Pole's double fault and an unforced error gave an early break to Rybakina, who then held her serve with an ace to go 3-0 up in the second set.

Wimbledon champion Swiatek was left frustrated when she made three successive unforced errors as Rybakina broke again to go 5-1 up, sealing the set in her favour.

Swiatek continued to struggle, making 17 more unforced errors as Rybakina handed her a bagel in the third set.

Australian Open champion Keys made 12 double faults, four times more than her opponent as Anisimova bounced back from a first-set stumble to get her first win in the WTA Finals.

Anisimova was the first to hold her serve after the match started with four straight breaks, but the 24 year-old conceded another break as Keys took a 5-4 lead.

World number seven Keys won the first set with forceful serves and built a 2-0 lead in the second, but her struggles with double faults and unforced errors allowed Anisimova to get a break back and make it 3-3.

Fourth seed Anisimova broke again, fooling Keys with her well-timed forehand, and won five straight games to take the second set.

Keys looked exhausted as Anisimova, growing in confidence and finding success with cross-court shots, took just 34 minutes to win the third set.

"It was quite a battle out there. Just really happy with the way I was able to turn around in the second set and turn that frown upside down," said Anisimova, who was the runner-up in Wimbledon and the U.S. Open this year.

Rybakina will face Keys in her last round-robin match, while Swiatek, who beat Keys on Saturday, takes on Anisimova.

In the other group, top seed Aryna Sabalenka, who beat Jasmine Paolini in her first match, plays Jessica Pegula on Wednesday. Pegula will also be seeking a second successive round robin win following her victory over Coco Gauff on Monday.