American Sloane Stephens produced a dominant final set to beat US Open champion Naomi Osaka at the WTA Finals.
The pair traded several breaks of serve in the first set but Stephens, 25, edged it 7-5 in 52 minutes.
Osaka, 21, rallied and closed out a 55-minute second set 6-4.
Japan’s Osaka had a break point when 1-0 down in the decider but failed to challenge a call when Stephens appeared to double-fault, and her rival surged to a 7-5 4-6 6-1 win.
Osaka’s serve deserted her in the final set as she was broken three times.
In all she produced 46 unforced errors, compared with 31 from Stephens – Osaka’s immediate predecessor as US Open champion.
Stephens, who later said the surface at the Singapore venue was providing “weird” bounce, joins Czech player Karolína Pliskova and Ukraine’s Elina Svitolina as victors in their opening matches of the WTA Finals.
The event features the leading eight players this year, split into two groups of four.
The top two players from each round-robin group will progress to the semi-finals, with the final on 28 October.
Germany’s Angelique Kerber takes on Dutch player Kiki Bertens later on Monday.