World number two Iga Swiatek overcame a shaky start to defeat Wimbledon champion Marketa Vondrousova in straight sets in her opening WTA Finals match in Cancun.
In Monday's other clash, US Open champion Coco Gauff opened her tournament with a 6-0, 6-1 drubbing of Tunisia's Ons Jabeur.
Polish star Swiatek looked out of sorts in an error-strewn first set in which she trailed 2-5 before roaring back to clinch a 7-6 (7/3), 6-0 victory.
The 22-year-old four-time Grand Slam champion is bidding to reclaim her world number one ranking from Aryna Sabalenka at this week's season-ending finals in Mexico.
However that goal looked unlikely during a nervy opening from Swiatek, who was broken in her first service game before seventh seed Vondrousova held for a 2-0 lead.
Swiatek managed to claw the break back in the fourth game to square the first set at 2-2 but then fell to pieces as Vondrousova broke for a 3-2 lead and then held to go 4-2 up.
Another shaky service game from Swiatek ended in a break of serve to leave Vondrousova in the driving seat at 5-2.
But Vondrousova's game collapsed thereafter, and Swiatek broke back twice in quick succession to level the set at 5-5 before holding for 6-5.
Although Vondrousova steadied the ship to hold serve and set up a tie break, it was only a temporary reprieve.
Swiatek quickly took control of the breaker and clinched the set when Vondrousova double-faulted.
Vondrousova's service game went into meltdown in the second set, with Swiatek breaking her three times in a row.
Another Vondrousova double-fault left Swiatek 5-0 up, and the Pole sealed victory on her serve in the final game.
"In the first part of the match I wasn't feeling like I was playing bad," Swiatek said.
"I just felt like I was making mistakes on the last shots within the rally. I felt there was room for improvement but I didn't need to change a lot.
"So I just tried to be more precise, not play risky."
The 19-year-old Gauff raced through the first set against Jabeur to win 6-0 in just 23 minutes and then shrugged off a lengthy rain delay to take the second set 6-1 for a convincing win.
"Ons is a great player, so I felt really unsteady the whole match even though it didn't look like that," Gauff said.
"But I'm just happy to finally get a win here in this event."
The WTA Finals sees eight players divided into two groups of four playing in a round-robin first phase, with the top two in each group advancing to the semi-finals on Saturday.
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