Mickelson, seeking his second win in as many weeks, shot a 5-under 66 over the Riviera Country Club course to lead J.B. Holmes and Hunter Mahan by a single stroke. Holmes jumped into contention for the first time this year in his return to competitive golf after undergoing brain surgery in 2011.
Mickelson won his 40th PGA Tour event last week at Pebble Beach, firing a spectacular 64 in the closing round to make up six shots on 54-hole leader Charlie Wi. In addition, Mickelson beat Tiger Woods by 11 shots in their head-to-head duel during the final 18 holes.
Mickelson backed up that effort Thursday by putting together an almost flawless round on a course toughened by gusting winds. Only 11 sub-70 scores had been turned in when play was suspended by darkness with 30 players still on the course.
The only blemish on Mickelson's card came at the par-3 16th, where he missed the green, chipped to 7 feet and failed to make the putt.
He came to the 475-yard, par-4 18th tied with Holmes and Mahan for the top spot and put his tee shot into the right rough, 200 yards from the putting surface. He hit his second shot just over the green and holed it from there.
Holmes had a chance to take the lead by himself, but he double bogeyed the par-4 eighth -- his 17th hole of the day.
Jonathan Byrd and Carl Pettersson were tied for fourth at 3-under 68 while those at 2-under included K.J. Choi, Brendan Steele, Brandt Jobe, Sergio Garcia, Matt Kuchar, Stewart Cink, Rory Sabbatini and Troy Matteson.
Sabbatini and Matteson were unable to finish their rounds before darkness fell.
World No. 1 Luke Donald shot a 70 and was tied for 14th place. Kyle Stanley, the early leader in the year-long FedEx Cup points race, had a 74. Stanley is playing for the first time since a two-week stretch in which he lost a big lead Sunday only to make up a large deficit seven days later to win his first tour event.