Mickelson knocked in birdies on five of the last six holes at TPC Scottsdale, ending the day with the second-lowest 54-hole total in PGA Tour history.
He also matched Mark Calcavecchia's 2001 tournament record with the effort, which came after he barely missed carding a 60 in Thursday's first round.
Mickelson is seeking his third Phoenix Open title and 41st win of his career.
Ending the day in second place at minus-18 was Brandt Snedeker, who registered a 6-under 65.
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