Mickelson opened up a considerable lead over the field before bogeying three straight holes. He birdied his final hole of the day to get back to 10-under par.
Casey fired a blistering 7-under 64 in tough conditions at Monterey Peninsula Country Club's Shore Course. His round marks the lowest of players who have completed their second round.
Jordan Spieth also sits at 10-under with two holes remaining to complete his second round at Spyglass Hill. He birdied five of his first seven holes but has since made nine straight pars.
Round 2 is set to resume Saturday at 10:10 a.m. ET. Round 3 is scheduled to get underway at 11:00 a.m. ET.