For a full two sets and nine games, Roger Federer was the picture of poise, picking up half-volleys with vintage panache and placing winners past Andy Murray.
Then, in the bounce of a ball - or two bounces of the ball, depending on opinion - the 17-time major winner was transformed back into the vulnerable, 2013-edition Federer.
After breaking Murray to get a chance to serve for the match last night and a spot in the Australian Open semi-finals for an 11th consecutive year, Federer dropped serve.
He recovered and jumped to a 5-2 lead in the tiebreaker, but then Murray rallied and won six of the last seven points to prolong the quarter-final.
After wasting a handful of break-point chances in Murray's next service game, which lasted almost 19 minutes, Federer finally wore down the Wimbledon champion 6-3, 6-4, 6-7 (6-8), 6-3 to set up a semi-final showdown with top-ranked Rafael Nadal.
"For me it was just a matter of staying calm and forgetting about it a little bit because, you know, the match was great until that point," Federer said.
Murray made his resurgence after disputing a point in the ninth game of the third set, when he thought the ball had bounced twice before Federer hit it. Replays shown in Rod Laver Arena were inconclusive. Federer thought was good, and left the call to the umpire.
"I'm proud of the way I fought," Murray said. "I've come a long way in four months."
But a day after Novak Djokovic's three-year Melbourne reign was halted by Stanislas Wawrinka, Nadal flirted with disaster in a 3-6, 7-6 (7-3), 7-6 (9-7), 6-2 win.
"You lose a little bit the co-ordination. Yeah, that's a big deal," Nadal said of his grip. "I served slower. I served bad, but I was able to win a match against a very difficult opponent, so that has much more value than when everything is great.
"And because of these victories, sometimes happens that next day you are able to play much better - and these victories are more important than the days that you are playing great."
And that is how the greatest of modern day champions reconcile it. On a day when two-time defending Australian Open champion Victoria Azarenka lost 6-1, 5-7, 6-0 to No 5 Agnieszka Radwanska of Poland and followed other stars out of the tournament, Nadal and Federer won back-to-back matches on centre court to set up their 33rd showdown.
There was an air of resignation rather than shock when Azarenka tamely followed Serena Williams and Maria Sharapova out of the tournament.
Silent assassin Radwanska toppled a shrieking Azarenka 6-1, 5-7, 6-0 to end the Belarusian's bid for a hat-trick of titles and reach her maiden Australian Open semi-final.
The crafty Pole frustrated Azarenka with a scrambling, cat-and-mouse game early and completely dismantled her in the final set to close out one of her finest victories in two sun-drenched hours at Rod Laver Arena.
"I'll be fine tomorrow. I'll be working tomorrow. It's not the end of the world. But I'm not happy with what I did today," said Azarenka.
It was a major scalp for Radwanska, who had lost the last seven meetings with Azarenka, and now goes into a semi-final with Slovak 20th seed Dominika Cibulkova. Li Na faces Eugenie Bouchard in the other semi-final.
"It's hard to play someone I lost to so many times before. I knew she's a great player. Especially here, she was playing amazing tennis," said Radwanska.
Agence France-Presse, Reuters