Five-time Major winning Phil Mickelson is now in doubt for next week’s Shell Houston Open after being forced out with a back injury after 10 holes on day three of the Valero Texas Open.

The reigning Open Champion showed no signs of being in any pain prior to leaving the course in a golf buggy and with his daughter sitting on his lap.

However he briefly advised officials he was struggling with an oblique muscle strain in his back.

Mickelson, who was one over par at the time, headed straight past the TPC San Antonio clubhouse and to his vehicle parked in front of the JW Marriott Hotel.

He had made the halfway cut near 7pm local time on Friday in playing a superb shot from a hazard to a few feet for a birdie.

However this is now the second occasion this year Mickelson has withdrawn from a PGA Tour event after pulling out of the third round of the Farmers Insurance Open in January because of back pain.