Coco Gauff posted a rare victory over Iga Swiatek to punch her ticket to the semi-finals of the WTA Finals in Riyadh on Tuesday, ensuring Aryna Sabalenka will finish the year as the world number one.

Swiatek, the defending champion, was in a direct battle with Sabalenka for the top spot this week but needed a win over Gauff to keep her chances alive of leapfrogging her Belarusian rival.

Gauff, 20, entered the contest having lost 11 of her 12 previous meetings with Swiatek, with the American's sole victory coming in Cincinnati last year.

But she turned the tables on Swiatek on Tuesday, defeating the second seed 6-3, 6-4 to make it two straight-sets wins from two round-robin matches in Riyadh this week.

"It feels great. I knew going into the match that despite our head-to-head I had a lot of confidence going in," said Gauff, who hit 11 double faults during the one-hour 48-minute showdown.

With the exception of the opening game of the match, Gauff had break point opportunities in every Swiatek service game in the first set.

The American third seed made her move in game seven, breaking for 4-3, and she closed out the set with a second break of serve, taking the lead in 49 minutes.

Swiatek needed six break points in a marathon third game of the second set to finally take down the Gauff serve but she couldn't consolidate her advantage.

They traded breaks again, as Gauff's double-fault tally went up to double digits and Swiatek racked up the errors.

It was Gauff who eventually steadied the ship, and wrapped up the victory on Swiatek's 47th unforced error of the match.

Swiatek, a five-time major champion, admits her two-month break between the US Open and the WTA Finals and the fact she is playing her first tournament with her new coach Wim Fissette have made things "a bit more tricky and different than usual".

"But I'll force myself to work even better in the next match," the Pole added. (AFP)