Coco Gauff beat defending champion Iga Swiatek 6-3 6-4 at the WTA Finals in Riyadh on Tuesday to reach the semifinals and ensure Aryna Sabalenka would finish the year as world number one.

It was Gauff's second win over Swiatek in 13 meetings. Gauff, 20, is unbeaten in the orange round-robin group having defeated fellow American Jessica Pegula in the previous round.

Swiatek can only qualify for the semifinals if she beats Pegula and Gauff also defeats Barbora Krejcikova on Thursday.

"It feels great. I knew going into it despite our head to head I had a lot of confidence. I felt like I was playing great tennis," Gauff said.

Gauff cruised through the opening set without facing any break points, while Swiatek’s 32 unforced errors proved costly. Swiatek rebounded briefly in the second set as she went up a break twice but Gauff swiftly regained control each time.

Despite Gauff’s nine double faults in the second set, it was Swiatek’s increasing errors that ultimately made the difference, with Gauff steadying herself to seal the victory on Swiatek's 47th unforced error of the match.

"Even when I was playing a bit sloppy, the games I lost were still going to deuce. They gave me confidence. I knew if I could stay solid, I’d have the chance to close out the match," Gauff added.

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"In the last game I missed two forehand returns in the net and I told myself  'it's ok I'll get the next one' and I did."

French Open champion Swiatek lost in the US Open quarterfinals in September and was leapfrogged in the world rankings by Sabalenka after she skipped the WTA Tour's Asian swing due to fatigue. Gauff secured the title at the China Open in Beijing and reached the semifinals in Wuhan.

Swiatek had dominated Gauff in their previous meetings, but the 23-year-old Pole looked rusty on her return after a two-month layoff, securing a comeback win over Krejcikova in her opening match in Riyadh.

Wimbledon champion Krejcikova's 6-3 6-3 win against Jessica Pegula earlier on Tuesday ended the sixth-seeded American's chances of qualifying for the semifinals in Riyadh.

Eighth-seeded Krejcikova, who retired from the Ningbo Open quarterfinals last month due to a back injury, bounced back from her opening 4-6 7-5 6-2 loss to Swiatek in the orange group to keep her hopes of reaching the final four alive.

"I was fighting for every ball and knew I had to play my best tennis. So, I was trying to play that and be really solid to put as many balls to the other side as I could," Krejcikova said on court.

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Pegula suffered her second straight-sets defeat of the week and will play her final round-robin match against Swiatek on Thursday.