Kiki Bertens won a remarkable deciding third set against Angelique Kerber – which included seven consecutive breaks – to make an impressive start on her debut singles appearance at the WTA Finals in Singapore.
The Dutch player is the lowest seeded in the eight-player field but held her nerve to hold serve to close out a 1-6 6-3 6-4 victory in just over two hours.
Bertens, who qualified for the season-ending tournament through Simona Halep's withdrawal, made 45 unforced errors but her greater aggression, which contributed to 33 winners, saw her shock the German top seed.
Bertens was a doubles finalist last year in Singapore
"I'm really happy to get the win here today," Bertens said.
"It was a slow start but I was really happy to turn around this match. I had a little chat with my coach when it wasn't going so well, so we decided to go a little bit more for my shots, play a little more aggressive and it worked out."
Kerber made the fast start in the Red Group meeting as she secured back-to-back breaks of serve before sealing the first set on her third opportunity.
Kerber appeared on course for victory leading by a set and a break
The three-time major winner appeared to increase her command on proceedings when she broke Bertens at the start of the second set but her 26-year-old opponent hit back to break twice to force a third set.
Bertens, who was a doubles finalist last year in Singapore, snapped a run of service breaks with a vital hold from 0-40 in the eighth game of the final set before a forehand winner earned the world No 9 a tour-leading 11th victory over a top 10 player this season.
Sloane Stephens earlier produced a dominant third-set performance as she opened her campaign with victory against Naomi Osaka in a match between the last two US Open champions.
Sloane Stephens secured her second victory over Naomi Osaka
The 2017 winner defeated the fellow debutant 7-5 4-6 6-1 in two hours and 24 minutes in Singapore.
Stephens - French Open finalist this season - mixed her trademark solid defence and counterpunch strategy to frustrate her inconsistent Japanese opponent.
"I just competed really well and never gave up," Stephens said.
"I knew I would have to play some really good tennis to beat her, stayed tough and tried to stay as positive as I could and took my opportunities when they presented themselves."
The match saw the past two US Open champions meet
Osaka came into the match as the form player on tour, with 14 victories from her previous 16 matches, but after the pair traded breaks in succession twice during the early stages it was Stephens who eventually gained the initiative on her third set point.
The 21-year-old displayed her frustration, with a shrugging gesture, when she squandered four break points in the fourth game of the second set but recovered to break at the next opportunity to move 4-2 ahead.
Osaka came into the event as the in-form player
Despite seeing her serve broken once again when she served out for the second set, Osaka forced the match into a deciding set when her opponent double-faulted on a second set point.
But the deciding set was one-way traffic as Stephens' greater consistency saw her break Osaka, who made 46 unforced errors in total, three further times to clinch victory.