Mickelson shot a 2-under 70 after opening with a 77 at TPC San Antonio's AT&T Oaks Course. Lefty was 11 strokes behind leader Steven Bowditch, the Australian who had a 67 to reach 8-under 136.
Mickelson was in the stream that runs in front of the 18th green after attempting to reach the par 5 in two from 288 yards with a 3-wood.
Bowditch holed out from 83 yards for an eagle on the par-4. The 30-year-old topped the leaderboard at the end of a round for the first time in his PGA Tour career.
Chad Collins and rookie Andrew Loupe were a stroke back. Collins shot 66, and Loupe had a 70.