Former champion Jason Day recovered from an early deficit to beat Ryder Cup hero Graeme McDowell 3&2 in their group match in the WGC-Dell Match Play.
But the world number two then headed straight into the clubhouse for treatment after appearing to injure his lower back while hitting his tee shot on the 16th hole at Austin Country Club.
McDowell, who holed the winning putt at Celtic Manor in 2010, holed from five feet for birdie on the third and was conceded the fourth after Day found trouble off the tee.
Good use of the backboard.
— PGA TOUR (@PGATOUR) March 23, 2016
G-Mac, 1-up. https://t.co/0NpEZzl6Ii
However the Australian hit back by driving the green on the par-four fifth to set up a winning birdie.
Jason Day just drove the 370-yard fifth.
— PGA TOUR (@PGATOUR) March 23, 2016
370 yards. On. The. Green. 👀 https://t.co/ERYAs8sfCt
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 in 2013, found water off the tee.
Day also birdied the 12th to move three up and pars thereafter were enough to seal victory, but the 28-year-old looked in considerable discomfort following his tee shot on the 16th and had to climb gingerly into a fairway bunker to hit his second shot.
In the same group, Thailand's Thongchai Jaidee had beaten England's Paul Casey 2&1, while there were also narrow victories for Charl Schwartzel over Danny Lee and Brandt Snedeker against Charley Hoffman.
World number one and Texas native Jordan Spieth was three up after six holes against 2014 Ryder Cup hero Jamie Donaldson, while Phil Mickelson had raced into a five-hole lead at the turn against Matt Fitztpatrick.
Players and caddies were wearing black ribbons in tribute to the victims of the terrorist attacks in Brussels, with Belgium's Thomas Pieters enjoying an early lead on former world number one Adam Scott in their group six contest.
