Jessica Pegula upset Aryna Sabalenka in straight sets at the WTA Finals in Cancun on Tuesday, punishing an error-strewn performance by the world number one to all but guarantee a semi-final berth.

World number five 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.

Depending on the outcome of Tuesday's other first round group game between Elena Rybakina and Maria Sakkari, Pegula could be assured of reaching the last four as winner of the Bacalar Group.

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.

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.

rcw/bsp