Singapore - A confident Sloane Stephens stormed to victory with a commanding third set performance against Japanese sensation Naomi Osaka as the upsets continued 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.
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.
"I competed really well and I knew I had to play good tennis to beat her," Stephens, the fifth seed, said after the match. "I just stayed tough and took my opportunities."
It was a much-anticipated showdown between the past two US Open champions, who were making their WTA Finals debuts.
Since her stunning upset of Serena Williams at Flushing Meadows, Osaka's popularity has surged and she had the crowd on her side at the Singapore Indoor Stadium.
It was 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, 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.
The 21-year-old was unable to close out the set on serve, blowing a forehand drive volley and dropping her racquet in disgust. But she redeemed herself by breaking straight back to level the match.
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.