Harrington still doesn't know all the nuances, although that didn't seem to matter on a spectacular day of sunshine and shotmaking. He opened with three birdies before the sun climbed over the tree-lined fairways. He started his back nine with four straight birdies. It led to an 8-under 63 that gave him a three-shot lead over Phil Mickelson, and plenty of praise from his peers.
"It's a little humbling to shoot what I thought was a good round and then get lapped," Mickelson said after a bogey-free 66 put him a tie for second with Briny Baird and Pat Perez.
Only after the round was over did Harrington realize he played more aggressively than he should have, hitting driver to the most narrow sections of the fairway, going after a pin at No. 15 on top of a steep shelf, only to have his 5-iron bang off the flag and settle 4 feet away.
Maybe ignorance was bliss.
Aside from that social game and two practice rounds, Harrington and Riviera are still getting to know each other.
"Some day it will catch up on me, the fact I don't know the golf course," the Irishman said. "Today it didn't. And whenever I did miss a shot, it seemed to be OK. There's a lot of pitfalls on this golf course. I played nicely, holed the putts, got the breaks, everything you need to do to shoot 63."
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.
Lefty has never finished in the top 10 in eight tries at this tournament, but he got off to a flying start in more ways than one.
He is staying at home this week in Rancho Santa Fe, just north of San Diego. He has a one-hour commute, not uncommon in these parts, only his is slightly different. He drives to the Palomar Airport in Carlsbad, takes a private jet to Santa Monica, then drives to Riviera.
"Left the house at 5:15 a.m. and got here at 6:15 a.m.," he said.
Jim Furyk, Sergio Garcia and David Howell were among those in the group at 67, while Vijay Singh and U.S. Open champion Geoff Ogilvy were among those at 68. Despite dry conditions and relative calm, the course still averaged about one shot over par.
Howell was among the late starters and was surprised to see 8 under next to Harrington's name.
"This is a great golf course," he said. "And that is a hell of a round."
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.
Harrington has been around long enough not to get overly excited about one round, especially the first one. And the more he learns the course, the more his strategy might change.
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.
"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 te 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.
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."
At this rate, Harrington is likely to keep coming back each year.