SINGAPORE — A confident Sloane Stephens stormed to victory Monday with a commanding third set performance against Japanese sensation Naomi Osaka to highlight another night of stunning upsets at the WTA Finals in Singapore.

In an unpredictable encounter, the American overcame a second set stutter to dominate the decider and triumph 7-5, 4-6, 6-1 in two hours and 25 minutes.

It was followed by another marathon match with number eight seed Kiki Bertens overcoming top-seed Angelique Kerber in a thrilling 1-6, 6-3, 6-4 comeback victory.

The lower seeds have won all four matches to start the WTA Finals.

Before Monday, Osaka had won 14 of her previous 16 matches to be the in-form player coming into the season finale.

But the third seed was wildly inconsistent in the Red Group opener with 46 unforced errors and was broken three times in a third set meltdown.

Stephens handled the warm conditions better to record an impressive victory.

“It’s really humid inside the stadium, so I played a little bit different,” she told reporters.

“Obviously playing someone with a lot of confidence is always a tough task, but I think that I played well and I kept a good attitude.”

Osaka lamented her error-strewn performance.

“I think I made more unforced errors than her in the third set,” she said.

“I don’t really think you can do that against her, so definitely I think that’s what made the difference.”

It was a much-anticipated showdown between the past two US Open champions, who were making their WTA Finals debuts.

The game got off to a sluggish start with both players struggling to hold serve and few winners being struck before Stephens made her move, aggressively rattling Osaka with deep groundstrokes and pinpointing the lines.

A subdued Osaka could not unleash her powerful game and grew frustrated as Stephens drew first blood.

Osaka lifted her intensity in the second set and rediscovered the form that has propelled her to a career-high world ranking of four.

It was only the sixth time this year Osaka has played in a three-set match but the unforced errors returned in a one-sided final set.

Stephens won the pair’s only previous match two years ago in Acapulco.