California: American world number eight Dustin Johnson withdrew from the Humana Challenge at La Quinta, California, midway through second round due to a lower back injury.
Johnson, the highest-ranked player in the field, had opened with a level-par 72 in his first competitive event since having arthroscopic surgery on his right knee in November.
"My knee is okay," the 27-year-old said in a statement after covering the first nine holes of his second round in four-over 40 at La Quinta Country Club, one of three venues hosting the pro-am celebrity event.
"I am not worried about it. My lower back is really stiff and hurting which I think is from the way I am walking or the fact that I have not walked a lot since the surgery.”
"Yesterday was the first time I walked a full 18 holes. I do intend to play next week in San Diego," he added, referring to the January 26-29 Farmers Insurance Open being played at Torrey Pines.
Johnson, a five-times PGA Tour winner, is one of the biggest drawcards on the U.S. circuit because of his long-hitting game.