Marana - England's Ian Poulter became the first major upset at the $9 million World Golf Championships Match Play Championship, losing to American Rickie Fowler 2 and 1 on Wednesday.
Poulter, who won the Match Play crown in 2010, reached the semi-finals last year before losing to Hunter Mahan.
But the world number 15 crashed out in the first round this year at the hands of Fowler, who birdied to win the fourth and took the par-3 sixth and par-four seventh with pars to lead early on 3-up, then topped Poulter's birdie at 14 with an eagle to win the hole.
"I was definitely an underdog coming in," Fowler said. "Heck, if I was filling out a bracket I would have picked Poulter too."
Fowler, a 14th seed in the quarter of the draw topped by Northern Ireland's Rory McIlroy, advanced to a second-round match against the hottest player on the US PGA Tour, fellow American Jimmy Walker.
Walker, seeded sixth, eliminated South African 11th seed Branden Grace 5 and 4. Walker birdied three of the first five holes and won the eighth with a par then closed out with a 12-foot birdie putt at the 14th.
"Just kept the ball right out in front of me and tried not to get in too much trouble," Walker said. "Giving yourself a lot of looks at birdie is key."
Walker has won three PGA titles already this season, capturing the 2013-2014 opener at the Frys.com Open, adding the Sony Open in Hawaii last month and taking the Pebble Beach National Pro-Am just 10 days ago.
Upsets became the order of the day with Swedish 15th seed Peter Hanson ousting US second seed Dustin Johnson 4 and 3 and South African 14th seed George Coetzee eliminating US third seed Steve Stricker 3 and 1.
Stricker made four bogeys on the front nine and Coetzee opened with 13 pars before birdies on two of his last three holes sealed victory.
Next for Coetzee is American 11th seed Patrick Reed, who beat Canada's sixth-seeded Graham DeLaet 1 up.
Reed's five-foot par putt at 18 was enough to seal the win after DeLaet missed a 40-foot birdie bid at the last.
Next up for Hanson is Frenchman Victor Dubuisson, a seventh seed who beat US 10th seed Kevin Streelman 4 and 3.
Bubba Watson, coming off his first victory since the 2012 Masters last week at Riviera, opened with a 2 and 1 win over Finland's Mikko Ilonen.
The US third seed next plays Swedish 11th seed Jonas Blixt, who ousted US sixth seed Keegan Bradley 2 and 1.
Results on Wednesday from the first round of the $9 million World Golf Championships Match Play Championship (x denotes seed):
Bobby Jones division
Henrik Stenson (SWE x1) bt Kiradech Aphibarnrat (THA x16) 2 and 1
Louis Oosthuizen (RSA x8) v Nick Watney (USA x9) 1 up
Brandt Snedeker (USA x4) bt David Lynn (ENG x13) 20 holes
Webb Simpson (USA x5) bt Thongchai Jaidee (THA x12) 3 and 2
Jason Day (AUS x2) bt Thorbjorn Olesen (DEN) 2 up
Billy Horschel (USA x10) bt Jamie Donaldson (WAL x7) 6 and 5
George Coetzee (RSA x14) bt Steve Stricker (USA x3) 3 and 1
Patrick Reed (USA x11) bt Graham DeLaet (CAN x6) 1 up
Ben Hogan division
Rory McIlroy (NIR x1) bt Boo Weekley (USA x16) 3 and 2
Harris English (USA x9) bt Lee Westwood (ENG x8) 5 and 3
Charl Schwartzel (RSA x4) bt Kevin Stadler (USA x13) 3 and 2
Jim Furyk (USA x5) bt Chris Kirk (USA x12) 2 and 1
Sergio Garcia (ESP x2) bt Marc Leishman (AUS x15) 22 holes
Bill Haas (USA x7) bt Miguel Angel Jimenez (ESP x10) 4 and 3
Rickie Fowler (USA x14) bt Ian Poulter (ENG x3) 2 and 1
Jimmy Walker (USA x6) bt Branden Grace (RSA x11) 5 and 4
Gary Player division
Justin Rose (ENG x1) bt Scott Piercy (USA x16) 1 up
Ernie Els (RSA x8) bt Stephen Gallacher (SCO x9) 19 holes
Jason Dufner (USA x4) bt Scott Stallings (USA x13) 19 holes
Matteo Manassero (ITA x12) bt Luke Donald (ENG x5) 5 and 4
Matt Kuchar (USA x2) bt Bernd Wiesberger (AUT x15) 3 and 2
Ryan Moore (USA x7) bt Joost Luiten (NED x10) 1 up
Jordan Spieth (USA x3) bt Pablo Larrazabal (ESP x14) 2 up
Thomas Bjorn (DEN x6) bt Francesco Molinari (ITA x11) 2 and 1
Sam Snead division
Richard Sterne (RSA x16) bt Zach Johnson (USA x1) 5 and 4
Hunter Mahan (USA x8) bt Gonzalo Fernandez-Castano (ESP x9) 3 and 2
Graeme McDowell (NIR x4) bt Gary Woodland (USA x13) 19 holes
Hideki Matsuyama (JPN x5) bt Martin Kaymer (GER x12) 2 and 1
Peter Hanson (SWE x15) bt Dustin Johnson (USA x2) 4 and 3
Victor Dubuisson (FRA x7) bt Kevin Streelman (USA x10) 5 and 4
Bubba Watson (USA x3) bt Mikko Ilonen (FIN x14) 2 and 1
Jonas Blixt (SWE x11) bt Keegan Bradley (USA x6) 2 and 1