AUSTIN, TEXAS >> Jason Day overcame an early deficit to win his opening match Wednesday in the Dell Match Play.
Now he has to overcome a bad back.
In the latest in a series of injuries for Day, he was reaching for his back for a couple of holes until he winced and grabbed it after a tee shot on the 16th hole. He took a knee on the green, where he closed out Graeme McDowell, 3 and 2.
Day headed straight for the fitness trailer for treatment.
The severity of the injury was unclear. Day left without speaking to reporters, and his agent later told the PGA Tour there would be no statement.
Top-seeded Jordan Spieth, the crowd favorite at Austin Country Club who helped Texas to a national title in 2012, had the largest crowd and delivered a solid performance. Spieth was 4 up through seven holes, and outlasted Jamie Donaldson, 3 and 2.
Rickie Fowler, the No. 5 seed, wasn't so fortunate. Fowler rallied from 3 down on the back nine to square his match on the 15th hole, only for Jason Dufner to win the next two holes with birdies in a 2-and-1 victory.
Dustin Johnson (No. 8) lost to Robert Streb, and Hideki Matsuyama (No. 12) lost to Rafa Cabrera-Bello.
In the round-robin format, all of the players still have a chance to advance from their groups and reach the knockout stage. A year ago, only three players who lost in the first round managed to reach the round of 16. It might be easier this year because there are halves during group play.
Bubba Watson (No. 4) halved his match with PGA Tour rookie Patton Kizzire.
Phil Mickelson, playing this World Golf Championship for the first time in five years, had the shortest match. Mickelson ran off three birdies in a four-hole stretch on the front nine to build a big lead and Matthew Fitzpatrick couldn't catch up. The match ended on the 14th hole.
Most of the drama on a windy day involved Day, who was coming off a victory Sunday in the Arnold Palmer Invitational.
Day's career has been filled with injuries. He withdrew after one round in the 2012 Masters with an ankle injury. A wrist and thumb injury kept him out for much of the spring in 2014 after he won the Match Play. He suffered vertigo symptoms at the U.S. Open last year. He took three months off at the end of last year to be with his wife when she gave birth to their second child and to concentrate on his health.
The back pain grabbed when the match was in hand.
Day lost two straight holes early in the match, including the par-3 third when he came up well short of the green and into a hazard. But he kept smashing tee shots and converting them into birdies, ending the back nine with two birdies for a 1-up lead. McDowell found the water on the 11th, and Day went 3 up through 13.
Lacking on a warm, blustery day were the number of great shots typically seen in match play. Part of that was Austin Country Club, unknown to just about everyone except for Spieth and Dufner, whose coach lives in Austin.
"I think everybody is kind of feeling out how it's going to be this week," Dufner said.
Mickelson took the last two weeks off and never knows how quickly his form will return. He looked like the same guy who has been contending at most tournaments.
"It was playing very difficult with the wind and the speed of the greens and contour," Mickelson said. "I hit a lot of good tee shots in play that allowed me to play fairly progressively into the greens. That was the key. I really didn't make any mistakes."