Jessica Pegula upset Aryna Sabalenka in straight sets at the WTA Finals in Cancun on Tuesday, punishing an error-strewn performance by the world No 1 to reach the semifinals of the season-ending tournament for the first time.
World No 5 Pegula, who had beaten Sabalenka only once before in five previous meetings, dominated her out-of-sorts Belarusian opponent from the outset to claim a 6-4, 6-3 victory.
Pegula was assured of reaching the last four as the winner of Bacalar Group later Tuesday after fourth seed Elena Rybakina defeated Greece's Maria Sakkari 6-0, 6-7 (4/7), 7-6 (7/2) in the day's other game.
"I feel more comfortable playing (top players) this year, I feel more confident," Pegula said after her win.
"I don't think it feels like such a big deal. Last year when I made top five, I think I psyched myself out a little bit. You have a couple thoughts in your head – do I really belong at that level?"
Sabalenka, who is one of several players who have complained about the condition of the court at this week's tournament, looked uncomfortable throughout against Pegula, finishing with 33 unforced errors and five double faults.
Sabalenka, who can still reach the last four with a win over Rybakina on Thursday, again bemoaned the state of the playing surface after her defeat, describing it as "like leather."
"You never know what's going to happen, whether the bounce is going to help you or work against you," Sabalenka said.
"I guess today was a day where it didn't work well for me. But it's okay I still have a chance to pass the group, so I'm trying to stay strong.
"I'll do some recovery tonight and just try to analyze what happened today. I'll do everything I can in the next match."
The 25-year-old from Minsk was broken in the fifth game of the first set and Pegula cashed in by holding serve in the next game to open up a 4-2 lead.
Sabalenka blasted a wild forehand wide in the next game to hand Pegula another break and leave her serving for the set at 5-2 up.
Although Sabalenka rallied to break Pegula in the next game before holding to cut the American's lead to 5-4, Pegula made no mistake to wrap up the first set in 39 minutes.
Sabalenka was soon deep in trouble in the second set, going a double break down to trail 4-0.
The game went with serve thereafter and Pegula was soon on the brink of victory at 40-15 and 5-2 up.
But back-to-back double faults by Pegula allowed Sabalenka off the hook, and the Belarusian went on to break to close to 5-3.
Yet any hope of an unlikely comeback ended in the next game, when another erratic service game left Pegula holding three match points at 0-40 up.
Sabalenka fought back to deuce, but was unable to hold, with Pegula closing out victory on her seventh match point.