Los Angeles - Jason Day's 3 and 2 victory over Graeme McDowell at the WGC Match Play on Wednesday left the world number two nursing a sore back that raised concerns not only for the rest of the week but for next months' Masters.
Australia's Day, who headed straight for treatment after signing his scorecard at Austin Country Club in Texas, was limping slightly coming off the 15th green and was clearly in pain when he dropped his driver after teeing off at the next hole.
He limped gingerly down the side of a fairway bunker to play his second shot at the par-five 16th, and was down on his knees alongside the green when Northern Ireland's McDowell missed a birdie putt that would have extended the match.
McDowell was 2 up through four holes after a birdie and a hole conceded by Day. But Day birdied four of the next five to seize control.
"I'm not going to say it rubbed salt in my wound when a guy is 3 up with three to go and then he starts hurting and is able to limp up and win the match, but it does put a salty edge in it for me," McDowell admitted.
The severity of the injury was unclear, but his agent, Bud Martin, issued a statement saying Day was preparing to play on Thursday.
Things went much more smoothly for world number one and top seed Jordan Spieth, a crowd favourite in his home state who won the first hole against Wales' Jamie Donaldson with a par and led the rest of the way en route to a 3 and 2 triumph.
"I felt comfortable, I really did," said Spieth, who birdied three holes in a four-hole stretch - winning two of those holes to build his lead to 4 up through seven.
In contrast to Spieth's front-running win, world number three and defending champion Rory McIlroy took the lead for the first time against Thorbjorn Olesen at the final hole for a 1 up win.
Northern Ireland's McIlroy trailed after 12 of the first 14 holes, but squared the match with a birdie at the 15th. At 18 Denmark's Olesen sailed his approach past the green and failed to get on the green with his next shot. He had to make a nine-footer for bogey, but McIlroy holed his three-footer for par to win the hole and the match.
"I didn't have my best stuff today, but I hung in there and hit good shots when I needed to," McIlroy said. "I felt like I played tee to green very well from 14 on."
Wednesday's matches were the first of three days of round-robin group play, with the winner of each of the 16 groups advancing after Friday to the knock-out stage of the $9.5m tournament.
The round-robin format is good news for a couple of highly seeded first-round victims - fifth seed Rickie Fowler of the United States and 12th-seeded Hideki Matsuyama of Japan.
Jason Dufner led most of the way en route to a 2 and 1 victory over Fowler, rolling in a 28-foot birdie putt at the 17th for the victory.
Dufner had taken a 3 up lead through 11 after his fourth birdie of the day. Fowler bounced back to square the match after Dufner three-putted 15, but Dufner birdied the next two to seal it.
"He made more putts than I did today, and that was probably the little bit of difference," Fowler said.
Spain's Rafa Cabrera Bello defeated Matsuyama 1 up, building a 3 up lead through nine and holding on after Matsuyama rallied to level the match. Cabrera Bello drained six-foot birdie putts at 16 and 17 to secure the win.
Five-time major winner Phil Mickelson made quick work of 21-year-old Englishman Matthew Fitzpatrick with a 5-4 win, and American Matt Kuchar thumped India's Anirban Lahiri 6 and 5 in the biggest rout of the day.
World Golf Championships Match Play round-robin scores on Wednesday at Austin, Texas (x-denotes seeding):
Jordan Spieth (USA x1) bt Jamie Donaldson (WAL) 3 and 2
Victor Dubuisson (FRA) bt Justin Thomas (USA) 3 and 2
Jason Day (AUS x2) bt Graeme McDowell (NIR) 3 and 2
Thongchai Jaidee (THA) bt Paul Casey (ENG) 2 and 1
Rory McIlroy (NIR x3) bt Thorbjorn Olesen (DEN) 1 up
Kevin Na (USA) bt Smylie Kaufman (USA) 2 and 1
Bubba Watson (USA x4) v Patton Kizzire (USA) halved
Emiliano Grillo (ARG) bt JB Holmes (USA) 3 and 2
Jason Dufner (USA) bt Rickie Fowler (USA x5) 2 and 1
An Byeong-Hun (KOR) v Scott Piercy (USA) halved
Adam Scott (AUS) v Thomas Pieters (BEL) halved
Bill Haas (USA) bt Chris Wood (ENG) 2 and 1
Matt Kuchar (USA) bt Anirban Lahiri (IND) 6 and 5
Justin Rose (ENG 7) bt Fabian Gomez (ARG) 2 up
Robert Streb (USA) bt Dustin Johnson (USA x8) 3 and 2
Kiradech Aphibarnrat (THA) bt Jimmy Walker (USA) 2 and 1
Phil Mickelson (USA) bt Matthew Fitzpatrick (ENG) 5 and 4
Patrick Reed (USA x9) bt Daniel Berger (USA) 1 up
Brooks Koepka (USA) bt Billy Horschel (USA) 3 and 2
Danny Willett (ENG) v Jaco Van Zyl (RSA) halved
Chris Kirk (USA) bt Branden Grace (RSA x11) 3 and 1
Russell Knox (SCO) v David Lingmerth (SWE) halved
Rafa Cabrera Bello (ESP) bt Hideki Matsuyama (JPN x12) 1 up
Kevin Kisner (USA) bt Soren Kjeldsen (DEN) 2 and 1
Sergio Garcia (ESP) bt Lee Westwood (ENG) 1 up
Marc Leishman (AUS) v Ryan Moore (USA) halved
Zach Johnson (USA x14) bt Marcus Fraser (AUS) 4 and 3
Martin Kaymer (GER) bt Shane Lowry (IRL) 1 up
Brandt Snedeker (USA x15) bt Charley Hoffman (USA) 2 and 1
Charl Schwartzel (RSA) bt Danny Lee (NZL) 1 up
Louis Oosthuizen (RSA x16) bt Matt Jones (AUS) 2 and 1
Andy Sullivan (ENG) bt Bernd Weisberger (AUT) 3 and 2