Austin - World number one Dustin Johnson and in-form Rory McIlroy suffered shock losses as the World Golf Championship Dell Technologies Match Play opened with a string of uspets on Wednesday.
Defending champion Johnson was spent spinning to a 3 & 1 defeat in his opening group game against Austria's Bernd Wiesberger, the world number 56 who is seeded 52nd for this week's tournament at the Austin Country Club in Texas.
Meanwhile McIlroy, who signaled his return to form on Sunday with a sensational victory at the Arnold Palmer Invitational, came back to Earth with a thud in his Group Six clash with Peter Uihlein.
Uihlein, the 57th seed, took control of the match over the front nine, notching three early birdies to reach the turn at 4-up.
A further birdie on the 10th took him 5 up, and although McIlroy staged a superb late rally with five straight birdies to trim Uihlein's lead to two heading to the 17th, the 28-year-old held on to claim a 2 & 1 win.
Uihlein later said he was relieved to have held off McIlroy's charge after the turn.
"It's very difficult - he turned it on late. I felt that if I kept trying to give myself a chance I could get it done," Uihlein said.
"You can't really control what he's going to do. It was like a boxing match - if you land a couple of jabs you assume he's going to land one eventually. But I was able to hold him off, which was nice."
The round-robin format the WGC Match Play has adopted since 2015 gives Johnson and McIlroy the opportunity to turn around their campaigns, with two more group games to come on Thursday and Friday before the start of Saturday's straight knockout rounds.
Elsewhere on Wednesday, Justin Thomas grabbed a 2-up win over Luke List in Group Two.
List was forced to putt for most of the round with a wedge after damaging his putter in frustration after a bogey on the sixth.
Thomas was made to sweat, however, after List won three holes on the back nine to move within one of halving the match.
Thomas, last year's PGA Championship winner, held on to seal his win with a birdie on the 18th.
"I was thinking how bad that would hurt if I couldn't get it done against someone putting with a wedge but he hit some great putts when he needed to," Thomas said.
World number four Jordan Spieth, meanwhile, needed a late charge after the turn to see off South Africa's Charl Schwartzel in their group match.
Spieth and Schwartzel were all square through 11 but Spieth edged into the lead on the 12th, when a penalty forced his opponent to drop before he eventually carded a bogey six.
That was followed by two quick birdies to put Spieth 3-up after 14 holes, helping set up a 2 & 1 victory.
In other first-round games on Wednesday, fifth seed Hideki Matsuyama got off the mark with a 2 & 1 defeat of compatriot Yusaku Miyazato.
Miyazato, ranked 53, had rattled Matsuyama with a blistering start, going 2-up after four holes thanks to a pair of early birdies.
However, Matsuyama went to the turn one up after winning three straight holes from the seventh onwards.
Miyazato made it all square with a birdie on the 10th, but two more birdies from Matsuyama on the 12th and 13th gave him a lead he was never to relinquish.
Results from Wednesday's opening group-stage matches at the World Golf Championships Dell Match-Play Championship at Austin, Texas (x denotes seed):
Bernd Wiesberger (AUT x52) bt Dustin Johnson (USA x1) 3 & 1
Kevin Kisner (USA x32) and Adam Hadwin (CAN x38) halved
Justin Thomas (USA x2) bt Luke List (USA x60) 2 up
Francesco Molinari (ITA x21) bt Patton Kizzire (USA x48) 3 & 1
Jon Rahm (ESP x3) and Keegan Bradley (USA x63) halved
Kiradech Aphibarnrat (THA x28) bt Chez Reavie (USA x43) 3 & 2
Jordan Spieth (USA x4) bt Charl Schwartzel (RSA x49) 2 & 1
Patrick Reed (USA x19) bt Li Hao Tong (CHN x34) 3 & 2
Hideki Matsuyama (JPN x5) bt Yusaku Miyazato (JPN x53) 2 and 1
Cameron Smith (AUS x46) bt Patrick Cantlay (USA x30) 2 up
Peter Uihlein (USA x57) bt Rory McIlroy (NIR x6) 2 & 1
Brian Harman (USA x18) and Jhonattan Vegas (VEN x44) halved
Sergio Garcia (ESP x7) bt Shubhankar Sharma (IND x62) 1 up
Xander Schauffele (USA x20) bt Dylan Frittelli (RSA x41) 1 up
Jason Day (AUS x8) bt James Hahn (USA x56) 4 & 2
Louis Oosthuizen (RSA x25) bt Jason Dufner (USA x42) 1 up
Kevin Chappell (USA x33) bt Daniel Berger (USA x26) 3 & 2
Ian Poulter (ENG x58) bt Tommy Fleetwood (ENG x9) 3 & 2
Paul Casey (ENG x10) bt Russell Henley (USA x51) 1 up
Kyle Stanley (USA x45) bt Matthew Fitzpatrick (ENG x31) 1 up
Julian Suri (USA x64) bt Marc Leishman (USA x11) 3 & 2
Bubba Watson (USA x35) bt Branden Grace (RSA x23) 5 & 3
Brendan Steele (USA x36) bt Charley Hoffman (USA x22) 1 up
Tyrrell Hatton (ENG x12) bt Alexander Levy (FRA x55) 3 & 2
Alex Noren (SWE x13) bt Kevin Na (USA x61) 4 & 2
Tony Finau (USA x29) bt Thomas Pieters (BEL x39) 2 & 1
Charles Howell (USA x59) bt Phil Mickelson (USA x14) 3 & 2
Rafa Cabrera Bello (ESP x17) bt Satoshi Kodaira (JPN x40) 2 & 1
Gary Woodland (USA x24) and Webb Simpson (USA x37) halved
Pat Perez (USA x15) and Kim Si-Woo (KOR x50) halved
Yuta Ikeda (JPN x47) bt Ross Fisher (ENG x27) 2 & 1
Matt Kuchar (USA x16) and Zach Johnson (USA x54) halved