Phil Mickelson pulled out of the Farmers Insurance Open at halfway after failing to shake off a back muscle problem.
The left-hander had struggled to a one-over 73 on the tougher South Course at Torrey Pines to close on two under - eight shots behind leader Jordan Spieth.
But despite that being enough to make the cut, the Open champion decided to withdraw from the tournament in California after seeking medical advice.
Mickelson, who also complained of back trouble when runner-up in Abu Dhabi last week, had told the Golf Channel that he was hoping to play through the pain over the weekend.
"I am a little bit torn right now on the best plan of action," he said after a bogey at his final hole. "I love this tournament and I want to play here. I love playing in San Diego, and this is a place where I grew up playing.
"And part of me also says I am getting into bad habits and if I hit it in this rough here, I've really got an issue because it's awfully hard to get it out and it jars my back."
Mickelson had looked uncomfortable from early in the first round when he shot a three-under 69 on the North Course. He said later his back "locked up'' and that "it's just tight and I just couldn't swing from the top."
The five-time major winner is due to defend his Phoenix Open crown in Arizona next week before heading back to California to attempt to win the AT&T Pebble Beach National Pro-Am for a fifth time.