Brandt Snedeker and Ted Potter Jnr. both top the leaderboard after day two of the Pebble Beach National Pro-Am.
The tournament, which is held across three different courses, pits professional players against amateurs over four days.
Snedeker carded a four-under-par 68 at Spyglass Hill, while fellow American Potter Jnr hit a 67 at Monterey Peninsula as the pair ended Friday on eight-under overall.
Scotland's Russell Knox, who started the day in the lead with Mahan, struggled on the Spyglass Hill course as a one-over-par 73 put him on five-under with five others.