SINGAPORE - Serena Williams regained her composure after smashing her racquet and being on the brink of defeat to beat Caroline Wozniacki 2-6 6-3 7-6 (6) on Saturday and reach the title match at the WTA Finals.
The best of friends off the court, the pair slugged it out like two prizefighters for more than two hours until Williams delivered the killer blow to end a contest filled with drama and escalating tension.
The world number one looked to be on the ropes after losing the first set and smashing her racquet in frustration, then falling 5-4 behind in the third set.
But Wozniacki, who lost to Williams in last month's US Open final, failed to serve out the match and the American made her pay, winning the tie-breaker.
Williams remains on course to win the elite season-ending championship for the third year in a row.
She will play either Simona Halep or Agnieszka Radwanska in Sunday's final at Singapore's National Indoor Stadium.