Day fought back from a sloppy front nine, during which he dropped a shot, before getting one back at the 11th and rattling off three consecutive birdies from the par-four 13th.
The Queenslander said the timing was frustrating but refused to let it stand in his way when he continues with a 7.5-metre par putt on the 16th on Saturday.
«It is disappointing as I was on a roll there, hitting some nice approach shots and holing putts,» Day said.
Five-time major winner Mickelson held a two-shot lead during his round but crumbled with three bogeys in his last four holes.
However a closing birdie kept him tied at the top of the leaderboard as he chases a fifth win at the Pebble Beach Pro Am.
«I’m lucky to finish the round because some guys [were] still out there playing in some tough stuff,» Mickelson said.
Curtis Luck was the next best Australian having teed off early and posted a hot round of 67 to climb to a six-under total.
Cameron Davis (four under), Matt Jones (71, two under), Aaron Baddeley (incomplete, two under), Adam Scott (72, one under), Ryan Ruffels (71, one under), John Senden (72, even par) and Rod Pampling (77, five over) round out the Australian contingent.