Woods put some life into a dreary afternoon of rain with four birdies over his last six holes for a 65 that left him tied for the lead when he walked off the course and fans chasing after him to cheer his charge.
It didnt last long, however, as Harrington ran off three birdies in a four-hole stretch to seize control. And even after the Irishman made a careless bogey on the par-5 16th, he answered with a 20-foot birdie on the 17th to restore his three-shot lead.
Of his 69 victories on the PGA Tour, Woods has only won five times when trailing by three shots or more after 54 holes. Two of those came earlier this year, when he made up a five-shot deficit at Bay Hill and a four-shot deficit at the Memorial.
Harrington was at 10-under 200.
Tim Clark also was tied for the lead early on the back nine. The rain made Firestone play even longer, and Clark had to hit longer clubs, which eventually caught up with him. It got even worse after his round, when Clark was penalized two shots for not replacing his ball mark on the 16th green. That gave him a 73.
Jerry Kelly had a 69 and was at 205. Ten players were another shot back, including the three major champions this year ' Angel Cabrera, Lucas Glover and Stewart Cink, who each had a 68.
A week before the final major of the year, this could be quite an opening act.
Harrington has virtually disappeared this season while searching for the right swing. He only recently found the answer and finally has seen some results. He has a 54-hole lead for the first time since the 2007 Irish Open.
Woods, already a four-time winner this year, is going after his record seventh victory at the Bridgestone Invitational and his 16th victory in a World Golf Championship.
It will be the first time Woods and Harrington have played in the final pairing since the 2006 Dunlop Phoenix on the Japan Golf Tour, which featured a rare collapse by the worlds No. 1 player. Woods blew a three-shot lead with six holes to play, then lost to Harrington on the second playoff hole.
Of his six victories at Firestone, Woods has trailed three times going into the final round.
This time, he was happy to even have a chance.
He missed a short par putt on the eighth hole for his only bogey and was six shots behind Harrington, who showed no indication he would come back to the field.
I figured if I could put half a dent in that, Id be all right, Woods said. At least Im there with a shot at it.
It looked as though he would do more than that.
He spun an approach back to 6 feet on the 13th for birdie, then picked up another birdie from 15 feet on the fringe at the 15th. His experience at Firestone really showed on the par-5 16th, where he faced a tricky shot from 90 yards to a back pin. His lob wedge hit some 8 feet short of the hole, took a hard hop past the hole, then spun back to a foot.
Harringtons shot was too hard and went into the back bunker, leaving him a downhill shot. He didnt reach the green and made bogey.
When the pin is back, you have all that green beyond the pond and it baits you into hitting it low, Woods said. Youve got to go in there high, and the first hop has always been springy.
He added his final birdie on the 18th, with just enough spin to trickle back to 7 feet.
Every putt I had for birdie, I made it, said Woods, who had only 23 putts. It was the complete opposite of yesterday.
The cheers resounded across Firestone, and it was obvious who ' and what ' they were for. Harrington didnt flinch, rolling in putts of 30 feet on the 12th, 20 feet on the 13th and hitting the flag with his tee shot on the par-3 15th, leaving him a 7-footer.
The Irishman will be going after his first World Golf Championship, and the chance could not have come at a better time. After what seemed like a lost year searching for the swing key to help him at impact, he goes to the PGA Championship next week at Hazeltine without having won since the PGA last year.
The pairings for the PGA Championship were released Friday evening. Harrington and Woods will be in the same group.