World number two, Jason Day defeated Northern Irishman Graeme McDowell in his opening pool match at the WGC-Dell match play at the Austin country club in Texas, despite suffering a back injury.

The 28 year old picked up the injury on the 16th tee and went for treatment after his 3&2 win.

In other matches, world number one Jordan Spieth dispatched Jamie Donaldson 3&2, defending champion Rory McIlroy overcame Thorbjorn Olsen on the last hole, while five-time major champion Phil Mickelson secured a 5&4 victory Matthew Fitzpatrick.

The five-day tournament moves from the round-robin format into a straight knockout at the weekend between the 16 group qualifiers.