(updates at end of Spieth's round)
By Martyn Herman
ST ANDREWS, Scotland, July 19 (Reuters) - Jordan Spieth sped through a gridlocked British Open leaderboard as a third straight major title loomed large but he had to share the spotlight with Irish amateur Paul Dunne at St Andrews on Sunday.
The 21-year-old American, bidding to become the first man to win the year's first three majors since Ben Hogan in 1953, began the delayed third round in the chasing pack but rattled in seven birdies for a six-under 66 to reach 11 under.
Dunne, a year older but inhabiting a different golfing planet to Masters and U.S. Open champion Spieth, ended a remarkable day of low scoring in a three-way tie for the lead on 12-under 204 with Australian Jason Day and 2010 British Open champion Louis Oosthuizen.
Not only is he on course for the amateur's silver medal after a nerveless 66, but he could yet become the first amateur to win the British Open since Bobby Jones 85 years ago.
At this most unusual of Opens, anything seems possible.
With flags hanging limp like damp tea towels -- a huge contrast to Saturday's gales that wiped out the third round and forced a first Monday finish at the British Open since 1988 -- an eclectic mix of major winners, amateurs and nearly-men tucked into the Old Course birdie banquet with relish.
The exception was Spieth's fellow American Dustin Johnson, the overnight leader, who collapsed to the day's second worst round of the day, a miserable three-over 75.
Apart from a bogey five at the ninth, Spieth was clinical.
The Texan responded to that setback with three consecutive birdies at the 10th, 11th and 12th and sunk another at the 15th before safe pars at the final three holes -- smiling and joking his way round with Sergio Garcia who is nine under after a 68.
"I kind of just wanted to stay patient today, let them come to me, and once I figured out my putting, it did," world number two Spieth, still on for an unprecedented calendar year grand slam, told a news conference.
Asked about the magnitude of what he is chasing, he added: "It hasn't come up in my head while I've been playing yet.
"I can't speak for tomorrow given it's the last round, and if I have a chance coming down the stretch, if it creeps in, I'll embrace it. I'll embrace the opportunity."
Dunne's bogey-free six-under 66 means he smashed the previous lowest 54-hole total for an amateur at the Open.
Incredibly, however, he was not the only player yet to join the paid ranks remaining in the hunt for the Claret Jug as American Jordan Niebrugge's 67 took him to nine-under.
"I saw that I was tied for first on the 10th and I said to my caddie how cool is it to leave the Open on Sunday, even if it's a different type of Sunday," Dunne said.
"The golf ball still does what you ask it to do, it's just there are more people watching."
Day, whose U.S. Open bid was scuppered by vertigo, made up five strokes during the day in a round of 67, as did Oosthuizen.
At various stages of a head-spinning afternoon with puts disappearing left, right and centre, five or six players shared the lead, including double Open champion Padraic Harrington who rolled in a 45-foot birdie at the 16th.
He carded a 65 to reach 10 under, although the day's best round was that of Marc Leishman whose eight-birdie 64 was just one short of matching the major record. He ended nine under.
Major winners Justin Rose, Adam Scott and Retief Goosen also put themselves in the mix for what promises to be a classic conclusion to the 144th Open.