Former champion Rory McIlroy crashed out of the WGC-Dell Technologies Match Play despite winning a match without hitting a single shot, while Shane Lowry is also out after defeat to Jon Rahm.
McIlroy, who lost his opening contest in group two to Denmark's Soren Kjeldsen, was due to face American Gary Woodland on Thursday, the man he beat in the 2015 final.
However, Woodland withdrew from the event at Austin Country Club due to a "personal family matter", leaving Kjeldsen - who would have played Woodland on Friday - needing only to halve his match with Argentina's Emiliano Grillo to advance to the last 16.
And Kjeldsen, who had never previously won a match in the tournament, won four holes in succession from the 10th on his way to a 4&3 victory to book his place in the knockout stages.
A statement released by Woodland's management company read: "Due to a personal family matter, Gary Woodland must withdraw from the World Golf Championships-Dell Technologies Match Play. We ask that you respect his family's privacy at this time."
Phil Mickelson remains firmly in contention for his first victory since the 2013 Open after storming into the last 16.
Mickelson thrashed Ryder Cup team-mate JB Holmes 6&5 at Austin Country Club on Friday to join Soren Kjeldsen and Alex Noren in the knockout stages of the £7.8m event.
The five-time major winner had previously beaten Si Woo Kim 5&3 and Daniel Berger 5&4 in group 14, meaning he has yet to play any of the last three holes in competition this week.
But the 46-year-old has the advantage of having conserved plenty of energy ahead of a possible four more matches, with the last 16 and quarter-finals scheduled for Saturday and the semi-finals and final on Sunday.
Masters champion Danny Willett received a welcome boost ahead of the defence of his title at Augusta by beating Scotland's Russell Knox 4&2, although Willett had already been eliminated.
Knox's defeat and a victory for Bill Haas over KT Kim ensured Haas and Kim would contest a sudden-death play-off to decide the winner of group 11.
Lowry meanwhile fell to a 2&1 defeat to Jon Rahm in Group 7 that ended his interest in the competition. After halving his opening game to Sergio Garcia, the Offaly man needed a win, but couldn't prevent Rahm from his second win on the bounce.
Lowry levelled the match with a par at the sixth, but Rahm retook the lead in the next hole, before Lowry ensured they were again all square going into the back nine.
The Spaniard retook the lead at the 12th and effectively sealed the match when Lowry couldn't equal his birdie at the par-5 16th hole.
A day after defending champion Jason Day pulled out of the event to spend time with his mother as she undergoes treatment for lung cancer, Italy's Francesco Molinari also withdrew with a wrist injury after suffering a 5&3 defeat to Thongchai Jaidee.
That meant Sweden's Alex Noren, who was due to play Molinari on Friday, was guaranteed to advance from group eight having beaten Bernd Wiesberger 3&2 to claim his second win of the week.
Phil Mickelson remains firmly in contention for his first victory since the 2013 Open after storming into the last 16.
Mickelson thrashed Ryder Cup team-mate JB Holmes 6&5 at Austin Country Club on Friday to join Soren Kjeldsen and Alex Noren in the knockout stages of the £7.8m event.
The five-time major winner had previously beaten Si Woo Kim 5&3 and Daniel Berger 5&4 in group 14, meaning he has yet to play any of the last three holes in competition this week.
But the 46-year-old has the advantage of having conserved plenty of energy ahead of a possible four more matches, with the last 16 and quarter-finals scheduled for Saturday and the semi-finals and final on Sunday.
Masters champion Danny Willett received a welcome boost ahead of the defence of his title at Augusta by beating Scotland's Russell Knox 4&2, although Willett had already been eliminated.
Knox's defeat and a victory for Bill Haas over KT Kim ensured Haas and Kim would contest a sudden-death play-off to decide the winner of group 11.
Willett (above) was eliminated with a game to spare after being thrashed 6&5 by Haas, while world number one Dustin Johnson made it two wins out of two by beating Martin Kaymer 3&2.
Two-time Masters champion Bubba Watson also showed signs of a return to form with his second win, the left-hander beating compatriot Scott Piercy 4&3.
Watson has not had a top-10 finish since September's Tour Championship and, despite being ranked seventh in the world at the time, was left off the United States Ryder Cup team.
But the world number 17 could be rounding into form at the right time with the Masters just a fortnight away.
"I'm always thinking about Augusta," Watson said. "If there's a major - not a golf tournament, but a major - built for me, where I have a good shot at it, that's it. That's just common sense, right? Everybody knows a lot of the holes go the way that I want to shape it, and there's not high rough."
No bogeys for Phil Mickelson today.
— PGA TOUR (@PGATOUR) March 24, 2017
Phil defeats J.B. Holmes 6 & 5 and advances to the weekend.#QuickHits pic.twitter.com/gprN0vRTDA
Watson can book his place in the last 16 by beating Thomas Pieters on Friday, the Belgian Ryder Cup star having lost to Jhonattan Vegas.
Two-time major winner Jordan Spieth had also been in danger of an early exit after losing his opening match, but the Texas native kept his hopes alive with a 4&2 win over Yuta Ikeda.
Lee Westwood was handed a win following Day's withdrawal and can force a play-off in group three by beating Pat Perez on Friday, while Kevin Na needs just half a point against Chris Wood after beating Justin Thomas in group six.
