World No2 Mickelson, though, bogeyed the sixth and fell to one under alongside Peter Hanson of Sweden, who had made the turn, and American Bubba Watson, after seven.
Former Open champion David Duval, in the wilderness since his 2001 victory at Lytham, began his day alongside Fisher at two under but he was quickly in trouble with a triple bogey at the par-three third, after his tee shot plugged underneath the lip of a greenside bunker.
Duval showed resilience by sinking a birdie at the next hole, the par-five fourth to get back to level par.
Defending champion, world No1 Tiger Woods, was also level par with five to play.
Today is the first time since 1983 that the US Open had extended into an extra day to complete 72 holes.