Harrington opened with three straight birdies. He ran off four in a row after making the turn, and even picked one up on the tough 15th hole when his 5-iron hit the pin and settled 4 feet away.
His only bogey on the back nine came from a three-putt on the par-3 14th, and the Irishman had a reasonable explanation for that.
"At this stage, I'm feeling invincible," he said. "I didn't think I was ever not going to make birdie."
It gave him a three-shot lead over Mickelson, Briny Baird and Pat Perez. Jim Furyk, Sergio Garcia and David Howell were among those in the group at 67 on a spectacular day of sunshine and shotmaking on the storied course off Sunset Boulevard.
Six players failed to finish the first round when it was too dark to continue; they will finish Friday morning.
Mickelson is playing for the first time since 2001 at Riviera, where he has never had much success. But coming off a five-shot victory at Pebble Beach where he tied the tournament scoring record, he kept right on rolling with a bogey-free 66. He hit only six fairways, but the rough is negligible this week, and Mickelson wasn't off by much.
His only disappointment was being three shots behind.
"It's a little humbling to shoot what I thought was a good round and then get lapped," Mickelson said.
The only other time Howell has played Riviera was two years ago, when it took four days to complete 36 holes because of rain. The Englishman found conditions quite different this year, and he was duly impressed _ by Riviera and by Harrington.
"This is a great golf course," he said. "And that is a hell of a round."
Vijay Singh started eagle-birdie, but a couple of late bogeys brought him a 68. Retief Goosen made only seven pars in his round of 71.
Mickelson played with Ernie Els, one of several international stars making their '07 PGA Tour debut this week. The Big Easy labored to keep the ball in the fairway, but escaped with enough clutch pars _ getting up-and-down from 90 yards on his final hole _ for a 69 that left him pleased, although tongue-tied.
He noted that Mickelson played "awful," then quickly corrected himself.
"Awesome. I think that's the word you use," Els said with a laugh.
For Harrington, call it blissful ignorance. He first saw Riviera a few years ago on his way to the Target World Challenge in December and loved it. But as he looked back over his round, he realized there were nuances to the course that he still hasn't learned, and was thankful he didn't pay for it in the opening round.
He played two practice rounds, and both times hit driver and a 6-iron on the 463-yard second hole, traditionally one of the toughest. But with the fairways running fast and firm, he was stunned to his see his drive land in the narrow neck of the fairway, leaving him only a 9-iron into the green to within 4 feet.
"If I had known I was going to do that, I wouldn't have been hitting my driver off the tee," he said.
Ditto for the 434-yard fifth hole, where the fairway ends after about 280 yards and drops down a shaggy hill. Harrington's tee shot went 277 yards, leaving him another short iron to 10 feet for birdie.
"There again is a hole that maybe I have to be a little less aggressive on for the rest of the week," he said.
But it worked out beautifully for him Thursday, although the Irishman is smart enough to pay for attention to the day of the week than the number on his scorecard.
"The good thing about shooting 63 is I'll be able to make some mistakes and still compete in this tournament," he said.
At this rate, he'll likely find Mickelson along the way.
They were on opposite sides of th golf course in the first round, starting out in crisp morning conditions that made the greens so firm it was difficult to find pitch marks until the sun and temperatures began to rise.
Even though it has been six years since Mickelson has been to Riviera, he still remembers a few tricks. With a back right pin on the short but tricky 10th hole, Mickelson still pounded a driver that landed on the green some 315 yards away and into a back bunker. His logic was to hit beyond the green, because it slopes to the back.
"Where I was, I at least had a chance to stop it," Mickelson said, and he left himself an 8-footer he made for birdie. "I haven't played here in (six) years, and it's pretty obvious you've got to get past the hole."
Someone told him that club officials might restore a ditch that once ran behind the green.
"Well, then, it looks like this one won't be back on the rotation," Mickelson said, pausing for effect. "Just kidding."
Divots:@ It was seven years ago this week that two-time Nissan Open champion Mike Weir first entered the top 50 in the world ranking. He didn't fall out of the top 50 until last week, slipping to No. 51. Weir opened with a 3-over 74 as he continues to struggle while going through swing changes. In four tournaments, he has yet to break 70 in the first round. ... Els says he probably won't have a U.S. base for when he plays the PGA Tour. He sold his house in Orlando, Fla., and had considered moving to South Carolina, but now says he will live off the golf course he is designing in the Bahamas.