In her opening WTA Finals match in Cancun, world number two Iga Swiatek overcame a rocky start to upset Marketa Vondrousova, the Wimbledon champion, in straight sets.
In the other match on Monday, Tunisia's Ons Jabeur was soundly defeated 6-0, 6-1 by US Open champion Coco Gauff to start her tournament.
In a first set marred by mistakes, Polish sensation Swiatek appeared out of sorts. However, she stormed back to win 7-6 (7/3), 6-0. In this week's season-ending finals in Mexico, the 22-year-old four-time Grand Slam champion is attempting to restore her world number one ranking from Aryna Sabalenka. 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.
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.
When Vondrousova double-failed, Swiatek immediately grabbed control of the breaker and sealed the set.
In the second set, Vondrousova's serve game collapsed as Swiatek broke her three times in a row.
Swiatek led 5-0 after another Vondrousova double-fault, and the Pole won the last game with a serve.