Bertens won a deciding set which saw seven breaks of serveKiki Bertens shocked Angelique Kerber at the WTA Finals after a deciding set that included seven breaks of serve.
Dutch player Bertens, 26, lost her serve twice as Kerber took set one 6-1, but turned the match in set two 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 being level following two sets that took one hour and 47 minutes, Stephens pounced to beat the US Open champion 7-5, 4-6, 6-1.
The deciding 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.
Bertens’ upset is the biggest of the finals so far as the lowest ranked player of the eight in the tournament overcame the highest.
Stephens is world number six but scored a final-set win over Osaka, who is ranked two places higherKerber – ranked number two in the world – ultimately wilted under the attack of the world number nine, who fired 33 winners, helping mask a dozen double faults.
Bertens is the first Dutch woman 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 features 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.