Coco Gauff upset defending WTA Finals champion Iga Swiatek for only the second time in 13 matches to reach the semi-finals in Riyadh, despite 11 double faults.
Former US Open champion Gauff defeated former world No 1 Swiatek 6-3 6-4 to improve to 2-0 in round-robin play and take control of the Orange Group on Tuesday.
It also ensures Aryna Sabalenka will finish the year as world No 1, with Swiatek no longer able to overtake the Australian Open and US Open winner.
The victory is Gauff's first over Swiatek since the 2023 Cincinnati Open and snapped the Pole's six-match winning streak at the WTA Finals.
"It feels great and despite our head-to-head I was still confident and I knew if I could find my game, I knew I had the chance to close out the match," Gauff told Sky Sports Tennis.
"It means a lot [the crowds]. You guys are holding it down and it was a really fun atmosphere. I like playing in atmospheres like this and it makes competing exciting.
"I was just trying to be resilient and play it deep. The conditions are tough and the altitude tough.
"I was trying not to give her too many unforced errors. It's just a game of cat and mouse in these conditions.
"You can't change the past. I'm in the process of changing things. There's going to be good days and bad days."
Both players made errors throughout, but Gauff proved the stronger in the end, letting out a huge roar after breaking serve in the final game.
The American finished the night with 10 winners to 33 unforced errors, 11 of which were double faults. Swiatek outpaced Gauff with 15 winners, but struck 47 unforced errors in a drama-filled encounter.
The Pole, playing just her second match under new coach Wim Fissette, can still join Gauff in the semi-finals but she will need to beat Jessica Pegula in her final match.
Skysports