Tiger Woods saw off the challenge of Rory McIlroy at the WGC-Dell Technologies Match Play, but could not get the better of Lucas Bjerregaard as the Dane went on to secure a place in Sunday's semi-finals.
Woods, three-up after McIlroy bogeyed the 11th hole, was pegged back to one-up going into the 16th after the Irishman birdied both the 12th and 13th.
But McIlroy missed the green and ended up carding a seven at the 16th to hand the initiative back to the American, who made the most of his advantage to clinch a 2&1 win after both players parred the 17th.
Woods then continued his momentum into the quarter-final against Bjerregaard, the tournament's number 50 seed, as three consecutive birdies saw the former world number one move two-up after five holes.
Bjerregaard, though, refused to be overawed, having won his last-16 clash with Sweden's Henrik Stenson 3&2.
The Dane fought back with a magnificent 35-foot eagle putt at the par-five 16th and another birdie made it all square heading into the last.
Woods went into the bunker, but managed to play out and have a four-foot putt for par.
Bjerregaard, however, was on the green in two and after Woods failed to make his par, the Dane took his chance to record a memorable 1up victory.
"We had a great match and I really enjoyed playing with him today,"
Bjerregaard said in an interview with The European Tour media.
"It was a cool experience for me, obviously to say I've beaten Tiger, there's not a lot of guys who can say that."
Matt Kuchar will await the Dane in the semi-finals on Sunday morning after the American saw off Sergio Garcia.
Garcia had been penalised for playing a second putt before it could be conceded by Kuchar on the seventh, but managed to regain some momentum after winning the 15th and 16th to be just one down.
After both players made par on the 17th, Kuchar held his nerve on the last to seal a 2up victory.
Speaking afterwards on Sky Sports, the Spaniard moved to draw a line under the issue.
Garcia said: "I screwed up, it is as simple as that. I obviously missed my putt and kind of tapped it with the back of my putter before he (Kuchar) said anything.
"It is a loss of hole, I understand that, the only issue was, Kuch was like 'I didn't want to take the hole, and do it like this'.
"I was like 'OK, so what do you want to do?' But obviously he didn't like any of the options, but it is fine. At the end of the day, I am the one who made the mistake."
Elsewhere, Open champion Francesco Molinari continued his fine form in the competition with an impressive 6&5 victory over Kevin Na after building on a rapid start.
The Italian will face Kevin Kisner, last year's runner-up, for a place in the final.
Kisner had earlier edged out Louis Oosthuizen 2&1.
South African Oosthuizen, the 2010 Open champion, missed the green at the par-three 17th and conceded to the American, who had sent his tee shot to within a foot for birdie.
McIlroy, meanwhile, posted an apology via his social media channels for not speaking to the media following his loss to Woods.
The four-time major champion uploaded a statement from Austin-Bergstrom International Airport, with the comment: "Onwards and upwards...." as he reflected on a "squandered" opportunity.
McIlroy said: "Definitely one I let get away but I'll learn and move on. I wish Tiger the best for the rest of the tournament, he made a couple of clutch putts on 15 and 17 to shut the door. See everyone at Augusta."
