Jelena Ostapenko was in fine form as she claimed her first WTA Finals victory, inflicting White Group winner Karolina Pliskova's first defeat of the competition by prevailing 6-3 6-1.
Ostapenko was already eliminated having lost her opening two matches against Garbine Muguruza and Venus Williams, while back-to-back triumphs had clinched top spot for Pliskova.
The Latvian struggled due to a high number of errors in her previous outings but she struck a series of impressive winners to get the better of the world number three.
It ensured Ostapenko ended a breakout year in which she won the French Open in victorious fashion, while Pliskova will need to pick herself up ahead of the semi-finals.
The Latvian was growing in confidence and went a break up in the second after a double fault from the Czech, and she did not look back from there.
Ostapenko claimed another break before serving out the match, a wide forehand from Pliskova ending the contest on the first of three match points.
Meanwhile, world number eight Caroline Garcia kept her WTA Finals campaign alive with an epic three-set victory over Elina Svitolina in Singapore on Wednesday.
Down 5-3 and on the brink of elimination, Garcia dug deep and reeled off four straight games to prevail 6-7 (9/7), 6-3, 7-5 in two hours and 44 minutes. It was the pair's second consecutive marathon following Svitolina's victory earlier this month in Beijing.
Garcia turned the tide through her high-voltage groundstrokes, enjoying a sizeable 58-33 advantage in winners. It means Garcia, Svitolina and Garbine Muguruza will battle for the final semi-final position from the Red Group with Caroline Wozniacki having booked a berth.
France's Garcia looked set to be eliminated after being broken in the eighth game of the final set but the Ukrainian was unable to serve it out. Garcia broke again and ran out with the contest.
In the first match of the evening, a rampant Wozniacki said a more assertive approach fuelled her spectacular start to the WTA Finals after she thrashed world number one Simona Halep in straight sets.
The 27-year-old stunned Halep 6-0, 6-2 in just 63 minutes in Singapore to storm into the last four of the year-end tournament for the loss of just four games.
