Roger Federer performed his second Houdini act of this year's Australian Open on Tuesday, saving seven match points en route to a nerve-wracking 6-3 2-6 2-6 7-6(8) 6-3 comeback victory over American world number 100 Tennys Sandgren.
Friday's third-round win against local hope John Millman, when the 20-times Grand Slam champion won six straight points from 8-4 down in the final set tiebreaker, was touted as the great escape.
His quarter-final victory four days later was more epic still.
Millman had a world ranking under 50 and had at least defeated the Swiss before, at the 2018 U.S. Open.
But Sandgren had never faced Federer, who in turn had never lost to someone as lowly-ranked as the American at Melbourne Park.
"You've got to get lucky sometimes," Federer said with a smile. "I tell you that because in those seven matchpoints you're not in control.
"I was just hoping that maybe he wasn't going to smash a winner, if he misses one or two, who knows what's going to happen? I think I got incredibly lucky today.
"... I don't deserve this one but I'm still standing here and I'm obviously very, very happy."
Federer started strongly, setting up an array of breakpoints before converting his sixth to go up 4-2 and take the opener, but his unforced error count kept piling up in the second as Sandgren, a quarter-finalist at the 2018 Australian Open, drew level.
Things got more messy for the Swiss in the third when he was broken early and then warned for obscene language, something that happens rarely to Federer on court.
Federer got involved in an argument with Serbian chair umpire Marijana Veljovic before taking a lengthy medical timeout that he later said was for a problem with his groin.
Sandgren had to wait almost nine minutes on court before Federer returned to resume the match but the American broke his opponent again, converting his sixth setpoint to nose ahead.
Sandgren, whose run to the last eight in 2018 was shrouded in controversy because of links to far-right activists on his social media account, hit 27 aces in the match and continued to put Federer's serve under pressure.
With his back to the wall, Federer shifted up a gear to save three matchpoints on serve, with the Rod Laver Arena Crowd gasping every time he survived.
He saved four more during the tiebreak to force a decider, amid delirium in the stadium.
Federer rode the momentum in the final set, breaking Sandgren's serve in the sixth game before converting his first matchpoint to set up a meeting against either Milos Raonic or Novak Djokovic for a place in Sunday's final.
"Draws are not getting easier. With these lucky escapes, you might play without expectations because you know you should be skiing in Switzerland," he said.
"Lucky to be here, may as well make the most of it. I better feel better than today otherwise I am really going skiing."