AUSTIN, Texas (AP) -- Jason Day was 3 up with four holes to play, an ideal start to the Dell Match Play that included five birdies and hardly any stress.
And then he felt his back tighten on a tee shot at the 15th hole.
It stung so badly on his next tee shot that Day grabbed his back and winced. He stepped gingerly into a bunker. When his short par was conceded, he took a knee. Exactly what happened was not entirely clear because Day immediately sought treatment and left without speaking. But with a long week ahead of him at Austin Country Club, and the Masters just two weeks away, this victory surely felt hollow.
Without much drama on the golf course because of tough greens and Texas wind, Day supplied it with the latest in a long list of injuries.
Jordan Spieth, a crowd favorite who helped Texas to a national title in 2012, produced what the crowd came to see. The No. 1 player built an early lead, made a few pivotal putts to stay in control and beat Jamie Donaldson of Wales, 3 and 2.
Rory McIlroy escaped with a victory. He never led until winning on the 18th hole when Thorbjorn Olesen shanked a chip. Martin Kaymer rode the wind and drove the green on the 376-yard 18th hole to wrap up a 1-up victory over Shane Lowry. Matt Kuchar had the shortest day, dispatching Anirban Lahiri in 13 holes.
Day overcame an early deficit and had the match in hand when he first felt pain. The severity of the injury was unclear. Day left without speaking to reporters, and his agent later sent a statement that he tweaked his back, had some work done on it, was pleased with winning and is getting prepared to play Thursday.
Day told the Australian Associated Press before getting treatment, "It's really hard to play with searing pain in both sides of your lower back. Just my back has seized up and I need to get inside right away for treatment and see what's wrong."
There was no need to worry about Spieth. He had the largest crowd and delivered a solid performance by going 4 up through seven holes.
"I felt like everybody was on our side today," Spieth said. "I almost felt like a home Ryder Cup match. And Jamie, I'm not sure what he would say about it. ... Everyone was very respectful, but every single step I took, somebody was yelling at you, 'Hook 'Em Horns' or 'Go Jordan, we're rooting for you.' Which is really, really cool."
Rickie Fowler, the No. 5 seed, 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 to beat him on the 17th hole. He was the highest-ranked played to lose.
Dustin Johnson (No. 8) lost to Robert Streb, and Hideki Matsuyama (No. 12) lost to Rafa Cabrera-Bello. Adam Scott (No. 6), who has lost seven straight matches in this event, had to settle for a halve with Thomas Pieters of Belgium. Players and caddies wore black ribbons in their caps to honor the victim of Tuesday's bombings in Brussels.
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) was among six players whose matches ended in a draw.
Phil Mickelson, playing this World Golf Championship for the first time in five years, 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. Matt Kuchar had the shortest match, a 6-and-5 victory over Anirban Lahiri, who flew to Texas from a runner-up finish in India.
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.
Day, coming off a victory Sunday in the Arnold Palmer Invitational, 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.