Then Woods stepped up a gear on the back nine, four birdies in his last six holes moving into a tie for the lead with Harrington and Tim Clark of South Africa at seven under par as the American posted a stunning five-under 65.

USPGA champion Harrington had spent most of his third round sparring with Clark in the final group out on the 7,400-yard South Course as rain fell throughout the afternoon.

The Dubliner began the day at seven-under, one shot clear of the South African and quickly stretched his lead to two shots after pitching from 110 yards to inside 10 feet as Clark slashed out of right rough to 30 feet, from where he two-putted the first hole for par.

Harrington sank the birdie putt and moved to eight-under.

American Scott Verplank had joined Clark at six-under with a birdie at the par-five second, but immediately gave back a shot with a bogey at the par-four third and slipped out of contention with five further bogeys over the next eight holes.

Clark birdied the second and third to draw level with the Irishman at eight-under.

Spain’s Miguel Angel Jimenez shot 66 to finish his third round at four-under, which was looking a very good score as bogeys started to litter the cards of the players in front of him.

Harrington and Clark both bogeyed the par-four fourth, while Verplank continued to fall away, but Kenny Perry reached four-under thanks to a 66.

So Harrington and Clark dropped to seven-under and saw their lead extended to three shots over the field.

Clark birdied the par-three seventh to move into the outright lead at eight-under as Harrington parred, but the South African fell back into a share of the lead with a birdie at the ninth as they both made the turn at seven-under-par.

Woods had begun steadily from two-under-par with four straight pars before a birdie at the par-three fifth was followed by bogey, birdie at the eighth and ninth.

Then Woods came alive on the closing stretch, birdies at the 13th and 15th moving the six-time Firestone winner to five-under, ominously close to the lead.

Even more ominous was his iron shot from 91 yards to inside a foot at the 16th, a smiling Woods strolling up to tap in for another birdie that took him to six under, a shot off the lead.

Jerry Kelly joined him there with his third birdie in four holes, at the 11th, but Woods was in the zone, his momentum carrying him to a closing birdie at the 18th and he walked off the final green to a huge roar from the rain-soaked gallery with a score of 65 at seven-under-par, tied with Clark and Harrington in the lead.

“I do feel comfortable here,” Woods said after his round. “As of right now, I’m tied for the lead but more than likely I’ll probably be one or two back going into tomorrow.

“This golf course, you’ve got to be very patient.”

Woods’ time at the top of the leaderboard was, as he predicted, short-lived as Harrington showed his mettle with back to back birdies, rolling in a 30-foot putt at the 12th and another of 21 feet at the 13th to pull two strokes clear.

Clark missed a seven-foot par putt at the 14th to drop to six under as Harrington parred the hole and the Irishman showed he was not the sort to be intimidated by Woods when he birdied the par-three fourth to move to 10-under-par, three shots clear.

Kelly dropped back to five under after bogeying the 13th while the three 2009 Major winners Angel Cabrera, Stewart Cink and Lucas Glover fired 68s, as did England’s Oliver Wilson, to all finish on four under alongside Perry and Jimenez.

They were also joined by Australian Robert Allenby, Americans Woody Austin and Zach Johnson and Canada’s Mike Weir, all of whom shot 69s.

American JB Holmes moved to three under with an early 65, while Ian Poulter bounced back from a 74 on Friday with a third-round 67 to reach two-under.