Jason Day has suffered a back injury in his opening round win over Graeme McDowell at the world matchplay golf event in Texas, potentially putting his Masters tilt in doubt.

The world number two was three-up after 14 holes when he winced in pain hitting a tee shot on the 15th hole, straining something in his lower back and suffering nerve pain.

He was able to scratch out halves in the next two holes to win the match 3 and 2 but was in serious discomfort, struggling to walk.

The Masters are just two weeks away.

Meanwhile, South African Charl Schwartzel has beaten New Zealand number one Danny Lee one up in their opening group match.