Iga Swiatek began her bid to regain the world number one ranking with a comeback success over Barbora Krejcikova at the WTA Finals.

Poland's Swiatek trailed the Wimbledon champion by a set and double break before regrouping to win 4-6 7-5 6-2.

This was the 23-year-old's first match since her quarter-final US Open exit in September and first under new coach Wim Fissette.

"At the beginning I felt a bit rusty but I'm happy that I found a way to play a bit more solid," said Swiatek.

She and Aryna Sabalenka are in contention to finish the year as the top-ranked player.

Defending champion Swiatek will need to win the title to replace Sabalenka, while the Belarusian needs to win all three round-robin matches, external or reach the final.

Coco Gauff takes on fellow American Jessica Pegula in the second Orange Group match later on Sunday.

The top two players from each group advance to the semi-finals. Sabalenka, Elena Rybakina, Jasmine Paolini and Zheng Qinwen are in the Purple Group and play on Monday.

The tournament is being held in Saudi Arabia for the first time - a move which has been criticised by some because of the country's human rights record.