Iga Swiatek began her bid to regain the world number one ranking with a comeback win over Barbora Krejcikova in the WTA Finals group stages.
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 or reach the final.
Sabalenka began her campaign with victory over Zheng Qinwen on Saturday.
Coco Gauff beat fellow American and doubles partner Jessica Pegula 6-3 6-2 in the second Orange Group match on Sunday.
The top two players from each group advance to the semi-finals, with Sabalenka, Zheng, Elena Rybakina and Jasmine Paolini in the Purple Group.
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.
French Open champion Swiatek took an extended break after the US Open, pulling out of tournaments in China for personal reasons before splitting with her coach of three years.
She arrived in Riyadh early to get the feel of the courts but her performance was scrappy in the opening stages of the match.
Broken in the first game, she had opportunities on the Krejcikova serve but her forehand in particular let her down.
Krejcikova, who finished outside the top eight in the world rankings but was given a place on account of being a Grand Slam champion, served out the set and quickly went a break up in the second.
Swiatek double faulted in her next service game to hand her Czech opponent a double break and gestured angrily at her box, before regaining her composure.
She cut down on the unforced errors to bring the set level at 3-3 and then benefited as Krejcikova threw in a horrible service game to concede the set.
Swiatek kept the momentum going into the decider and led an increasingly hampered Krejcikova 5-0 before sealing victory after two hours and 35 minutes.
‘Sometimes sloppy, sometimes great – but that’s tennis’
Image source: Getty Images
Image caption: Coco Gauff won the China Open title in October
Gauff’s US Open title defence ended in the fourth round in September but, after parting with coach Brad Gilbert, she immediately bounced back by claiming the China Open title.
The serve that let her down so badly in New York appeared sharper against Pegula, with the 20-year-old hitting two aces to just two double faults.
Her forehand, arguably her biggest weakness, was still error-prone against Pegula, but Gauff was more solid throughout.
She broke to go 4-2 up and then calmly served out the first set, wrapping it up with a forehand winner.
Pegula struggled for consistency against her good friend, often following up a well-constructed point with an uncharacteristic miss.
She was broken in the first game of the second set and whacked her racquet to the floor in frustration, but that seemed to reset her as she broke back from 40-0 down.
However, four unforced errors in a row from Pegula handed Gauff the break back, and the younger American promptly won four games in a row to clinch victory.
“Sometimes it was sloppy, sometimes it was great – but that’s tennis,” said Gauff.