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La Jolla, CA (SportsNetwork.com) - Phil Mickelson withdrew from this weekend's Farmers Insurance Open after completing Friday's second round while battling muscle pain in his lower back.

Mickelson, a three-time winner of this early-season event (1993, 2000, 2001), struggled to a 1-over 73 on Friday after shooting 3-under 69 in the opening round. The reigning British Open champion presently sits eight strokes back of tournament leader Jordan Spieth.

Spieth fired a 9-under 63 on Friday to take a 1-stroke lead over Stewart Cink entering Saturday's third round.

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