Roger Federer passed a five-set test from Jo-Wilfried Tsonga Wednesday to set up an Australian Open semi-final with Andy Murray, as Serena Williams was shocked by teenager Sloane Stephens.
The great Swiss needed all his coolness and concentration as he was twice pegged back from a set lead before taking it 7-6 (7/4), 4-6, 7-6 (7/4), 3-6, 6-3, reaching his 33rd Grand Slam semi-final.
"It was a tough close for sure, but the whole match was tough," he said. "Any set could have gone either way. It's tough because you never know what Jo's going to come up with."
Facing Federer across the net on Friday will be Britain's US Open champion Andy Murray, who hit top gear in his 6-4, 6-1, 6-2 win over unseeded Jeremy Chardy and is into the last four without dropping a set.
World number one Novak Djokovic, chasing the tournament's first title hat-trick of the professional era, plays indefatigable Spaniard David Ferrer on Thursday after all four top men's seeds reached the semis.
But in the women's competition, America's Stephens, 19, precipitated a result of seismic proportions when she knocked out a limping Williams 3-6, 7-5, 6-4 to qualify for her first Grand Slam semi-final.
Williams, unbeaten since August and on the verge of returning to world number one, had been widely expected to win her third straight major title and was even eyeing the first calendar-year Grand Slam since 1988.
But the 15-time Grand Slam champion seized up with back spasms in the second set and Stephens took advantage, setting up a semi-final against defending champion Victoria Azarenka on Thursday.
"This is so crazy," said Stephens, who once had a poster of Williams on her bedroom wall. "I think I'll put a poster of myself up now."
Williams, 31, said she was almost glad her tournament was over after rolling her ankle, whacking herself in the mouth with her racquet and finally suffering the strained back during a torrid time in Melbourne.
"Oh, my gosh, I'm almost relieved that it's over because there's only so much I felt I could do... oh my gosh, it's been a little difficult. I've been thrown a lot of balls these two weeks," she said.
Williams won the first set but at 3-4 in the second, she pulled up with back spasms and later called for the trainer. As she served at half-pace, Stephens was eventually able to capitalise and she levelled at a set each.
Williams recovered sufficiently to break in the tense decider but Stephens broke back and then broke again to claim the victory of her life.
"Today I just really didn't have anything to lose. I mean, you're playing for the semis of a Grand Slam. You just got to go out and do it really," Stephens said.
Stephens' next opponent is Azarenka, who came through a marathon, 1hr 17min opening set against resurgent Russian veteran Kuznetsova before completing her 7-5, 6-1 triumph in 1hr 47min.
"She played really well throughout the whole match," Azarenka said. "At the beginning, it took adjustment because she plays such a different game."
Elsewhere, Murray finally reached his peak performance as he impressively sent unseeded Frenchman Chardy spinning out.
Murray broke Chardy's service eight times and played a clean match with 32 winners against 20 unforced errors, as he extended his Grand Slam winning streak to 12 matches and reached his 12th major semi-final.
"Today was the best I've played. I've struggled in my last few rounds a little bit, my last opponent (Gilles Simon) was injured so it wasn't much of a match as he struggled physically," Murray said.
"Jeremy's had a great tournament, he's beat some top players so I needed to come out sharp and get off to a good start."
Friday's semi will be Murray's first Grand Slam meeting with Federer since last year's Wimbledon decider, when he wept freely in defeat. Murray gained some revenge shortly afterwards by beating Federer in the Olympic final.
