Houston - Five-time Major Champion Phil Mickelson remains in serious doubt for next week's Masters after informing officials on Tuesday he would not be available to compete in this week's Pro Am at the Houston Open.
Should the 43-year-old world number five miss the Masters it would see the first of this season's Majors lose a second major drawcard after earlier on Tuesday Tiger Woods said he would miss the event for the first time in his career after undergoing back surgery.
Mickelson, though, remains in the draw for the first two rounds of the $6.2million Houston Open which gets underway on Thursday.
The Masters starts the following week at Augusta National in Georgia.
Mickelson withdrew after 10 holes on day three of last week's Texas Open in San Antonio because of a muscle strain in his back.
He attended the NBA match between the San Antonio Spurs match and the New Orleans Pelicans later that night before returning home to San Diego for doctors to assess the injury.
It meant Mickelson missing out on playing planned practice rounds on Monday and Tuesday at Augusta National where he captured three of his five Majors.
But while Mickelson sought to play down the injury his remarks last Saturday to playing partner Stuart Appleby of Australia suggests the injury could be more serious.
"Phil had hit a few poor shots but it was the first hole of our back nine when I really noticed there was something wrong, as he just didn't look right," said Appleby.
"Then he sort of feathered one down the next and that was it. He then came over to John (Merrick) and myself and said: "Look guys. I think something's going on in my back and I'm out of here."
Mickelson also is expected to host a press conference ahead of competing in Houston, an event he captured by three strokes in 2011.
"We haven't heard a word from Phil, so we don't know whether he's playing or not," said a PGA Tour official.
"All we know at this stage is that we had a call saying he be excused from competing in the Pro Am."
Mickelson has been drawn to play the opening two rounds in Houston alongside fellow Major Champions Webb Simpson and Keegan Bradley.
Australia's Scott Gardiner is on standby as first reserve.