Kiki Bertens shocked Angelique Kerber at the WTA Finals after a deciding set that included seven breaks of serve.
Bertens, 26, lost her serve twice as Kerber took the first set 6-1, but turned the match in the second when much of Kerber’s poise disappeared.
And Wimbledon champion Kerber, 30, was broken four times in the decider as Bertens overcame three breaks on her own serve to win 1-6 6-3 6-4.
Earlier on Monday, American Sloane Stephens beat Japan’s Naomi Osaka.
After two tight sets that lasted one hour 47 minutes, Stephens ran through the decider to beat the US Open champion 7-5 4-6 6-1.
The third set was not without controversy as, trailing 1-0, Osaka failed to challenge a call that would have given her the second game when Stephens appeared to double fault.
And the 21-year-old paid for her 46 unforced errors, 15 more than Stephens, who later said the Singapore venue was providing “weird” bounces for players.
But Bertens’ surprise win is the biggest of the finals so far as the lowest ranked player of the eight in the tournament overcame the highest.
Germany’s Kerber – ranked two in the world – ultimately wilted under the attack of the world number nine, who fired 33 winners, helping compensate for a dozen double faults.
Bertens is the first Dutchwoman to break into the world’s top 10 since Brenda Schultz-McCarthy in 1996 but only entered the tournament for the top eight players in the world when Simona Halep withdrew through injury.
From the eighth game in set two, Kerber lost her serve five times in a row, leaving her 4-3 down on Bertens’ serve in the decider.
And Bertens celebrated wildly as she closed out, an outcome that looked highly unlikely after Kerber’s dominant display in the opening set.
The WTA Finals feature the leading eight players this year, split into two groups of four.
All eight have now played once and the top two players from each round-robin group will progress to the semi-finals, with the final on 28 October.