US seeds Bryson DeChambeau and Justin Thomas and four-time major winner Rory McIlroy were among the upset losers in Wednesday's opening group matches at the WGC Match Play Championship.
English 60th seed Ian Poulter, a WGC Match Play and World Match Play winner as well as a Ryder Cup stalwart, won the final five holes for a 6&5 rout of 11th seed McIlroy in the 64-player showdown at Austin (Texas) Country Club.
French 58th seed Antoine Rozner, who won at Qatar two weeks ago on a 60-foot putt at the 72nd hole, downed fifth-seeded DeChambeau 2-up thanks to a birdie at 18 while 52nd-seeded Matt Kuchar beat second-seeded countryman Thomas 3&2.
Unlike a normal match-play event, first-day losers still have a chance to advance from a group stage into weekend knockout matches.
Poulter won his 30th match in 15 appearances with birdies at the par-4 ninth and par-3 11th and an eagle at the par-5 12th before closing out McIlroy with a par at 13, his all-or-nothing attitude dominating.
"It gets the adrenaline going, the ticker starts going. You got to pony up and hit shots," said Poulter, whose lead fell to 1-up after McIlroy birdied six and eight.
McIlroy, who has started working with swing coach Pete Cowan, suffered his worst Match Play defeat in a decade - since an 8&7 loss to ben Crane in 2011 - and hooked his tee shot at the par-4 fifth out of bounds left into the swimming pool of a nearby home.
"There's a couple of areas of his game which I'm sure he wants to firm up a little bit," Poulter said. "He missed a couple of tee shots left. But it's Rory, it doesn't take a lot for Rory to spark up pretty quickly."
Rozner birdied the ninth to go 2-up on reigning US Open champion DeChambeau. The big-hitting American birdied the par-5 16th and forced an 18th hole, where the Frenchman landed his approach inside eight feet from the cup and made the putt for the win.
"I was just trying to play a good round of solid golf, not give any shot away," Rozner said. "It's the strongest field I've ever played in in my life and I played good golf out there."
Kuchar birdied four of the last five holes on the front nine for a 5-up edge and parred his way to victory from there.
Spanish third seed Jon Rahm edged Colombian Sebastian Munoz 1 up, sinking a birdie putt from eight feet at 18 after the South American sank one three times as long.
Of the top eight seeds, four halved and two lost, the winners being Rahm and world number one Dustin Johnson, who beat 61st-seeded US compatriot Adam Long 2 up.
Reigning Masters champion Johnson birdied the par-4 third and parred the par-4 fifth to lead 2 up. Long pulled level with five to play but Johnson sank a 25-foot birdie putt at 15 and one from just outside four feet at 18 to win.
South Africa's Dylan Frittelli, the 64th and last seed who lives only 15 minutes from the course, ripped US 12th seed Tony Finau 6&5. Frittelli was 4-up after six and didn't lose a hole.
"It's huge," Frittelli said. "I've told a lot of people if I can get through Tony with a win, I think I'm definitely the favourite in the group."
US 63rd seed JT Poston halved with US fourth seed Collin Morikawa.
Spain's Sergio Garcia jumped atop group eight with a 4&3 win over England's Lee Westwood after England's Tyrrell Hatton and Matt Wallace halved.
"I feel like I can be quite aggressive," Garcia said. "If we have a mess up we can move on and can try to get it back."
Results from Wednesday first day of group matches at the WGC Match Play Championship at Austin Country Club (x denotes seed):
Dustin Johnson (USA x1) bt Adam Long (USA x61) 2 up
Robert MacIntyre (SCO x41) bt Kevin Na (USA x28) 2&1
Matt Kuchar (USA x52) bt Justin Thomas (USA x2) 3&2
Kevin Kisner (USA x34) bt Louis Oosthuizen (RSA x22) 2&1
Jon Rahm (ESP x3) bt Sebastian Munoz (COL x56) 1 up
Ryan Palmer (USA x24) bt Shane Lowry (IRL x38) 4&2
Collin Morikawa (USA x4) and JT Poston (USA x63) halved
Billy Horschel (USA x32) bt Max Homa (USA x35) 1 up
Antoine Rozner (FRA x58) bt Bryson DeChambeau (USA x5) 2 up
Tommy Fleetwood (ENG x21) and Kim Si-woo (KOR x45) halved
Xander Schauffele (USA x6) and Andy Sullivan (ENG x57) halved
Scottie Scheffler (USA x30) bt Jason Day (AUS x44) 2 up
Patrick Reed (USA x7) and Bubba Watson (USA x55) halved
Joaquin Niemann (CHI x26) and Christiaan Bezuidenhout (RSA x33) halved
Tyrrell Hatton (ENG x8) and Matt Wallace (ENG x51) halved
Sergio Garcia (ESP x39) bt Lee Westwood (ENG x18) 4&3
Webb Simpson (USA x9) bt Talor Gooch (USA x59) 6&5
Mackenzie Hughes (CAN x48) bt Paul Casey (ENG x17) 3&2
Patrick Cantlay (USA x10) bt Brian Harman (USA x54) 1 up
Carlos Ortiz (MEX x42) bt Hideki Matsuyama (JPN x23) 4&3
Ian Poulter (ENG x60) bt Rory McIlroy (NIR x11) 6&5
Cameron Smith (AUS x25) bt Lanto Griffin (USA x46) 1 up
Dylan Frittelli (RSA x64) bt Tony Finau (USA x12) 6&5
Jason Kokrak (USA x29) bt Will Zalatoris (USA x40) 1 up
Kevin Streelman (USA x53) bt Viktor Hovland (NOR x13) 4&2
Abraham Ancer (MEX x27) bt Bernd Wiesberger (AUT x43) 3&2
Daniel Berger (USA x14) bt Erik van Rooyen (RSA x62) 6&4
Harris English (USA x19) bt Brendon Todd (USA x47) 1 up
Jordan Spieth (USA x49) bt Matt Fitzpatrick (ENG x15) 3&1
Matthew Wolff (USA 20) bt Corey Conners (CAN x37) 3&1
Im Sung-jae (KOR x16) bt Russell Henley (USA x50) 1 up
Victor Perez (FRA x31) bt Marc Leishman (AUS x36) 2&1