He is level with Phil Mickelson on 11 under, whose 65 included an eagle on the 17th at the Riviera Country Club.

Charles Howell, who finished second at the event in 2003, also carded a 65 and is at eight under, three off the lead.

Pat Perez, Spain's Sergio Garcia and Englishman David Howell are tied for fourth place on seven under.

Harrington, who was at eight under overnight, started his second round on the back nine and promptly bogeyed the 10th.

He birdied the next, then sandwiched two bogeys at 13 and 15 around a birdie at 14 as Mickelson was methodically making his way up the board.

But Harrington had backto-back birdies at 16 and 17 to make the turn at nine-under and then picked up two further shots at the 1st and 6th coming in.

"I'm happy enough, " he said. "It's always hard to follow up a good round of 63.

Anything in the 60s was really the goal.

"I'm still making mental errors here and there but I made some birdies to counteract some of the bogeys."

Harrington said he was looking forward to playing in the final group with Mickelson last night.

"It's a lot easier to play with a big, noisy gallery, " he said.

"It adds a bit of excitement."

Mickelson, who cruised to a five-shot victory at the Pebble Beach National Pro-Am last week, was grateful for his long eagle at the penultimate hole for hauling him level with Harrington.

He said: "I just flushed my approach. It came off perfect, ended up in the middle of the green and I made the putt."

Garcia's round included five birdies and two bogeys.

"I felt better yesterday, "