World No 2 Jason Day suffered an injury scare just two weeks before the Masters after recovering from an early deficit to beat Ryder Cup hero Graeme McDowell in the WGC-Dell Match Play.
Day headed straight into the clubhouse for treatment after appearing to injure his lower back in the closing stages of his 3 and 2 victory over McDowell in their opening group match at Austin Country Club, Texas.
The 28-year-old did not speak to reporters but his coach and caddie Colin Swatton said he first noticed an issue on the 15th tee.
Day also grimaced and immediately let go of the club after pulling his tee shot on the 16th and had to back gingerly into a fairway bunker before playing his second shot.
McDowell, who holed the winning putt for Europe at Celtic Manor in 2010, was two up after four before Day hit back by driving the green on the par-four fifth to set up a winning birdie.
Further birdies on the eighth and ninth took the US PGA champion into the lead and he doubled his advantage on the 11th when McDowell, who won the Volvo World Match Play title in 2013, found water off the tee.
Day, who won the Arnold Palmer Invitational on Sunday, also birdied the 12th to move three up and pars thereafter were enough to seal victory, but it remains to be seen if he will be able to continue in the event.
He could forfeit today’s match with Thongchai Jaidee and then hope to beat Paul Casey tomorrow, although that would put Jaidee in pole position to win the group after a 2 and 1 win over Casey.
World No 1 and Texas native Jordan Spieth beat Jamie Donaldson 3 and 2, while defending champion Rory McIlroy fought back from two down to be all square after 15 with Thorbjorn Olesen.
The only Scot in the field, Russell Knox, played out a square match with Sweden’s David Lingmerth after both missed birdie attempts at the last.
The biggest win of the early starters had come from Phil Mickelson, who did not lose a single hole in beating England’s Matt Fitzpatrick 5 and 4.
England’s Andy Sullivan defeated Bernd Wiesberger 3 and 2, while Jason Dufner edged out Rickie Fowler 2 and 1 thanks to birdies on the 16th and 17th.
Former Ryder Cup team-mates Sergio Garcia and Lee Westwood endured a rollercoaster contest which saw just three holes halved before Garcia emerged victorious on the 18th.
Garcia was two up after three before losing the next four holes in succession, only to then win four holes in a row himself from the ninth to gain some semblance of control.
Two former winners were preparing to tee it up in the Puerto Rico Open after failing to qualify for the Austin event.
Ryder Cup star Ian Poulter and former world No 1 Donald missed out after sliding down the world rankings.
Poulter slipped from 64th to 67th in the rankings after finishing joint 67th in the Valspar Championship.
The top 64 qualified for the Match Play, although with Henrik Stenson opting not to play and Jim Furyk ruled out through injury, the cut-off line moved to 66th.
That meant Poulter missed out by just 0.044 ranking points, although as first reserve he left his journey to Puerto Rico as late as possible in the hope that someone would withdraw in Texas.
Donald won the Match Play title the year after Poulter but did not qualify in 2015 and has slipped to 92nd in the current rankings after two seasons without a win.