Jordan Spieth surged 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 after a remarkable day's action 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 and lost ground early on but he stormed home with a seven-birdie, six-under 66 for an 11-under total, one off the lead.

Dunne, a year older but inhabiting a different golfing planet to Masters and U.S. Open champion Spieth, ended a crazy day of low scoring in a three-way tie at the top on 12-under par 204 with Australian Jason Day and 2010 British Open champion Louis Oosthuizen.

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Clearly on course for the amateur's silver medal after a nerveless 66, Dunne remains in serious contention to 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 since 1988 -- an eclectic mix of major winners, amateurs and nearly-men tucked into the Old Course birdie banquet with relish.

There were 37 sub-70 rounds, although the one notable exception was Spieth's fellow American Dustin Johnson, the overnight leader, who collapsed to the day's second=worst round, a miserable three-over 75.

SPIETH CLINICAL

Spieth struggled to make much headway early on and was clearly getting frustrated when he dropped a shot at the ninth.

But the Texan regained his already legendary composure to respond with three consecutive birdies at the 10th, 11th and 12th and sunk another at the 15th before pocketing 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 green 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 and more cameras."

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 Padraig Harrington who rolled in a 45-foot monster 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 with the top 25 players separated by five shots.

"There are so many players who can still win this," Oosthuizen said. "It's going to be I think one of the tightest Opens,"