Jelena Ostapenko

Venus beats Ostapenko at WTA Finals

The match, lasting 3 hours, 13 minutes, featured 20 service breaks, with neither player finding much confidence on their service games.

The 37-year-old Williams dropped her serve nine times, while the 20-year-old French Open champion conceded serve 11 times.

"I don't know if there was any luck for me tonight, I just had to work for every point,'' the fifth-seeded Williams said. "It's not easy to lose the first match, and both of us were in the same situation. What else can you do but fight to live another day?''

Ostapenko, Halep in Paris final

Ostapenko celebrated her 20th birthday by crushing Bacsinszky's hopes with a nerve-jangling 7-6 (7-4) 3-6 6-3 victory, which included a total of 16 breaks of serve on a sweltering Court Philippe-Chatrier.

But her impressive tally of 50 winners, including a bludgeoning forehand on her second match point, helped her become the youngest finalist at any major since Caroline Wozniacki in New York eight years ago.