Tiger Woods, the tournament host and winner in 2001, birdied the final hole to shoot his second 1-under 71 in as many days. He is at 2-under-par 142 and is tied for second place with 2000 champion and first-round leader Davis Love III (72) and Chris DiMarco (68).
'Considering the mistakes I've made the last two days, if you take those away, I'm well ahead,' admitted Woods. 'I'm surprised that no one else has run away with it. I'm very fortunate to be in there.'
Harrington caught fire quickly into Friday's round with three birdies in his first four holes. He struggled on the par-5 fifth and made bogey, but the winner of last week's Hong Kong Open on the European Tour waited until the back nine to ascend into the lead.
At the par-4 10th, Harrington rolled home a 25-footer for birdie to reach 1 under par, but the Irishman dropped the stroke with a bogey at the difficult, par-3 12th. He birdied the par-5 13th but once again dropped a stroke quickly after a birdie, this time at the 14th.
Harrington missed the green with his tee ball at the par-3 15th, but bladed a sand-wedge and rolled in a chip from 20 feet. He sank a seven-footer for birdie at the 16th from the fringe, then chipped in again from 10 feet at the 17th to reach 3 under for the tournament.
He missed his approach short on the lower tier of the 18th green. Harrington's 35-foot birdie try just missed, but the top-ranked European player in the world still held the 36-hole lead and is in position to become the first player to successfully defend his title at this event.
'I thought it would vault me into contention, but I'm surprised to be leading, to be honest,' said Harrington, who won this tournament at 20 under par last year. 'I like the golf course. It asks a lot. You can take it on.'
Woods found trouble very early in his round Friday at the par-5 second hole. His second went left into a hazard, then after a drop, his fourth hit the green, but spun back into another hazard. He walked off with a triple-bogey 8.
The No. 1 player in the world rebounded the rest of the way with six birdies and two bogeys to get into second place.
Love was in first place until some miscues off the tee on a pair of par-3s. At the 12th, Love missed the green and left his second in the rough. His third flew 15 feet past the hole and Love missed the putt for a double bogey.
He birdied 13 from 12 feet but his tee ball at No. 15 came up short, hit a rock and landed in water. His pitch stopped 10 feet from the hole and he rolled home the putt to save bogey. Love holed a two-footer for birdie at 16 to earn his piece of second.
DiMarco was 3 under on his round and one behind Harrington when he played the par-3 17th. His claw-handed putting stroke ran home a 35-footer for birdie and got him into Saturday's final pairing.
Robert Allenby (71), K.J. Choi (71), Justin Leonard (72), Nick Price (69), Masters champion Mike Weir (68) and 2003 PGA Tour's leading money winner Vijay Singh (69) share fifth place at 1-under-par 143.
Fred Couples shot an even-par 72 and is tied for 11th with reigning PGA Champion Shaun Micheel at plus-1. Micheel posted a 3-under-par 69 on Friday.
Kenny Perry, who shared second place at 1-under 71 after Thursday's opening round, struggled to a 5-over 77 and shares 13th with Jay Haas. The 49-year-old Haas shot a 72 in the second round and joined Perry at 1-over-par 148.
British Open winner Ben Curtis carded a 2-over 74 and is in 15th at plus-8, followed by Darren Clarke, who posted a second-round 73 to finish in 16th at plus-10.