Coco Gauff continued to be owned by Iga Swiatek as she went down 6-0, 7-5 to the Polish No. 2 seed at the WTA Finals on Wednesday (2 November).
The US Open champion lost to Swiatek for the ninth time in 10 career meetings - all in straight sets.
Swiatek - now 2-0 here in Cancun, Mexico, and top of the Chetumal Group - broke Gauff three times in the first set which she took in a swift 28 minutes.
Gauff was far more engaged in the second set, taking a 3-1 lead at one point. But the 19-year-old was unhelped by five double faults, four of them coming as she was serving for the set, before Swiatek pinned her down.
For the match, Gauff was broken six times and yielded 30 unforced errors. Her semi-final fate rests in her final round-robin tie against Wimbledon champion Marketa Vondrousova, who suffered her second defeat after losing 6-4, 6-3 to Ons Jabeur.
Swiatek faces Jabeur to close out the group phase.