Polish second seed Iga Swiatek moved within touching distance of the WTA Finals last four after a confident win against US Open champion Coco Gauff.
Gauff, 19, served for the second set at 5-4 but hit four double faults in a row to gift the break back to Swiatek.
Third seed Gauff, who can still qualify after thrashing Ons Jabeur, won one more point as Swiatek sealed victory.
“Coco is the kind of player who doesn’t give up so I knew it wouldn’t be that easy,” said Swiatek, who is aiming to regain her world number one ranking from Aryna Sabalenka at the season-ending event.
“The key is to stay focused on yourself. The conditions at the end were tricky so I was happy I could just survive and get through.
“You can’t predict which way the wind will go, it was swirling. It is uneasy but there is no reason to overthink it, you just have to adjust.”
The conditions were also set to be a factor later on Wednesday when Tunisia’s Jabeur faced seventh seed Marketa Vondrousova – in a repeat of this year’s Wimbledon final which the Czech won.
On Tuesday, American Jessica Pegula beat top seed Sabalenka in the other group to become the first player to reach the semi-finals.
Belarus’ Sabalenka plays Kazakhstan’s Elena Rybakina on Thursday to determine the second player to qualify from the group.
The Finals have been marred by the players criticising the court conditions at the venue, where the construction of the surface was only completed in the days before the tournament started on Sunday.
