Mickelson got hot with his irons on the back nine with back-to-back birdies inside 5 feet and a mammoth 3-wood from 287 yards that set up an eagle on the 17th hole, sending him to a 6-under 65 and a chance to win for the second straight week.
They were at 11-under 131, three shots clear of Charles Howell III.
Mickelson hasn't played Riviera since 2001, and he only added this tournament to his schedule at the last minute to test the newfound confidence in his driver. Two rounds has sent it soaring.
He reached all three par 5s in two, none more impressive than the 17th. Coming off his only bogey of the tournament, when he pulled his tee shot into a back bunker on the par-3 16th, Mickelson hit a bullet of a tee shot down the left side of the fairway some 310 yards. He followed that with a hard 3-wood that drew slightly and found the middle of the green, rolling to 15 feet.
'I just flushed it,' Mickelson said. 'It was a nice way to finish.'
It was another command performance, similar to last week at Pebble Beach when he won by five shots. He only missed four fairways, but none of them were off the fairway by much, and he never had a problem going after the flag.
He finished his round with an approach that sailed right of the green, but a flop shot landed softly and trickled 4 feet below the hole, allowing him to save par and go into another weekend in the final group. The next quiz comes on the weekend, for Mickelson has made the cut only five times in nine tries at the Nissan Open, and he has never finished in the top 10.
'It's a good start for two rounds,' Mickelson said. 'But one of the best players in the world, Padraig Harrington, is tied for the lead. The greens are going to be firm. It's going to be hard to get the ball close to the hole. It's going to be a challenge.'
Harrington found himself scrambling from the start, three-putting for bogey on No. 10 when he missed a 4-foot putt. But every mistake was followed by birdies on par 3s and good wedge play.
Howell was in the first group off and matched Mickelson for the best score of the day, playing bogey-free for a 65 to finish at 134. Howell made four straight birdies around the turn, two of them par 5s.
Sergio Garcia chipped in for birdie on the 17th on his way to a 68 and was in the group at 135 that included David Howell (68) and Pat Perez (69). Ernie Els didn't hit a fairway on the back nine until the 18th hole, and missed birdie putts inside 12 feet on his final two holes for a 68 that left him at 5-under 137, along with Jim Furyk (70) and Rich Beem (68).
Charles Howell III, the 27-year-old American, hasn't won in five years, but he took another step toward ending that drought. He already has two runner-up finishes this year, at the Sony Open and Buick Invitational.
'The best thing I can do is keep giving myself chances,' said Howell, who was at 8-under 134.
He gave himself plenty of chances around the turn, facing a three-hole stretch of two par 5s. Howell not only took care of the par 5s, he threw in birdies on the 18th with an 8-iron into 15 feet, and on No. 2 with an 8-iron into 10 feet.
'Riviera is an interesting golf course,' he said. 'There's a few birdie holes out here, and then all of a sudden you have a run of a few holes were par is a good score. So playing holes No. 17 and No. 1, you really have to make birdie on those holes.'
Garcia, among several international players making his PGA Tour debut at Riviera, finished strong. He traded birdies and bogeys most of the round until sticking his tee shot within 2 feet on the par-3 14th, holing a 10-foot birdie on the par-3 16th and chipping in on the 17th with a shot that quickly changed his mood.
He laid up on the par 5, but hung out his wedge to the right rough. But when the chip banged off the pin and disappeared into the hole, the Spaniard was all smiles.
'I just thought, 'What the hell? Why don't you just chip this in?'' he said. 'And it worked out.'
U.S. Open champion Geoff Ogilvy and former British Open champion Ben Curtis were among those at 4-under 138, while former Ryder Cup captain Tom Lehman finally showed signs of life with a 66 that put him at 139.
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