CANCUN, Mexico (AFP) — Jessica Pegula upset Aryna Sabalenka in straight sets at the WTA Finals in Cancun, 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 the 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.