Victoria Azarenka had to draw on all her reserves by saving two match points in a heroic three-hour six-minute tussle with Angelique Kerber in the WTA Championships.
The top-seeded Belarussian beat the German revelation 6-7 (11/13), 7-6 (7/2) 6-4 in an encounter full of magnificent rallies and surprises. Her win leaves Azarenka needing just one more win to guarantee her finishing the year as world number one.
Azarenka saved both match points by following up sound serving with bold and courageous driving. That helped make up for the five set points that Kerber had enterprisingly denied the top seed in the first set tie-break.
Had Azarenka lost either of them - from 4-5, 15-40 in the second set - a pathway might have opened up for Maria Sharapova to sneak through to the summit instead.
Azarenka would now have to lose both her remaining group matches for this to happen, even though Sharapova later made sure of a place in Saturday's semi-finals by beating Agnieszka Radwanska, the Wimbledon runner-up from Poland, 5-7, 7-5, 7-5 in another three-hour encounter.
"I felt I had to go for it on both match points," Azarenka said. "I felt she would not miss, so I didn't want to wait for her to make a mistake - I was determined to make it happen.
"I wanted to change the momentum and take destiny into my own hands. I had a clear mind of what I had to do. It takes time to go through (what's needed) and learn that.
"I feel really proud I was a part of this match. Honestly you feel like you don't want to leave the court, the atmosphere is so good!"
Azarenka's three-set encounter also guaranteed that Serena Williams' second success of the tournament earned the American a semi-final place.
The Olympic, US Open and Wimbledon champion won 7-6 (7/2), 6-3 against Li Na, the former French Open champion from China, but only after a noisy, and surprisingly fraught performance.
Williams had to overcome the emotions which impelled her to smash a racket and earned her a code violation warning.
That happened during a weird fourth game in which one of the finest servers in the history of the game delivered two successive double faults to go a break down.
"I guess I was angry and I wasn't able to control myself," she said. "But sometimes I play better when I get angry."
Hers was a triumph of will more than anything, for she landed less than 50 percent of her first serves, dropped service games five times, and needed an hour and 50 minutes to get the win.
"I lost serve today more often than all of Wimbledon," Williams said. "I was just thinking about it too much. I didn't serve well yesterday and I thought about it. I just need not to think.
"But to win with a zero serve, compared with what I normally do, is something I can take from the match."