Serena Williams, her insatiable will to win as strong as ever, 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 almost two and a quarter hours until Williams delivered the killer blow to end a contest filled with drama and escalating tension.
The World No 1 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, blinked first, missing her chance to serve out the match and the American made her pay by going on to win the deciding tie-breaker.
"Caroline played a great match, it was really difficult today," Williams said in a courtside interview.
"I feel so bad. She's the only one undefeated (after the group stages) in this tournament and she deserved to win."
Williams only snuck into the semi-finals on a countback after losing to Simona Halep in the round-robin phase but is suddenly back on track to win the elite season-ending event for the third year in a row.
She will play either Halep or Agnieszka Radwanska in Sunday's final at Singapore's National Indoor Stadium.
Wozniacki made a flying start to the match, reeling off the first three games for the loss of just three points.
Williams lost her opening service game when she hit a half-volley just over the baseline but held her next two, sealing her first game with a booming 190 km/h ace.
The American was unable to make any early inroads on Wozniacki's serve and her frustrations boiled over after she fell 5-2 behind.
She slammed her racquet into the purple hardcourts three times, destroying her equipment and incurring an official warning from the chair umpire Kader Nouni.
Unfazed by her opponent's outburst, Wozniacki immediately broke Williams again to wrap up the opening set in just 25 minutes.
Williams raised her game for the second set, eliminating the mistakes that cost her the opening set and putting Wozniacki under pressure.
She broke the Dane's serve for the first time with a ferocious return that Wozniacki chipped back then broke her again to level the match at one set apiece when Wozniacki double-faulted.
The third set lasted longer than the first two combined as both players traded blows and ran themselves close to exhaustion.
Wozniacki saved a break point midway through the third set after winning a 26-shot rally which saw both players darting from side to side.
Wozniacki, in training for next month's New York marathon, appeared to have the edge in fitness when she broke Williams in the ninth game but was unable to serve out the match.
Wozniacki saved a match point in her last service game to force the tie-breaker then two more after Williams had reeled off five points in a row.
But the American set up a fourth match point with a 198 km/h ace, her 12th of the match, then sealed the victory when Wozniacki hit a forehand wide.
