The Open: Amateur threatens to upstage Spieth’s treble bid

But the Masters and US Open champion could be denied a place in the record books by an equally remarkable performance after 22-year-old Irish amateur Paul Dunne claimed a share of the lead at St Andrews.

Days after being mistaken for Spieth by fans seeking autographs due to their identical clothing, Dunne carded a flawless 66 on the Old Course to finish 12 under par alongside playing partner Louis Oosthuizen – who won the 2010 Open at St Andrews – and Australian Jason Day.

Spieth also shot 66 to lie just a shot behind, with 2007 and 2008 champion Padraig Harrington on 10 under after a superb 65.

Nine players were tied for sixth on nine under, including another amateur in American Jordan Niebrugge, Justin Rose, Sergio Garcia and Adam Scott, along with England’s Danny Willett, who led outright after 10 holes but drove out of bounds on the 14th.

University of Alabama student Dunne – whose coach Alan Murray is also his caddie this week –is the first amateur since the legendary Bobby Jones to lead the Open after 54 holes, the American going on to lift the Claret Jug at St Andrews in 1927.

Three years later, Jones became the last amateur winner of the Open to date at Royal Liverpool and went on to complete the “Impregnable Quadrilateral” of Amateur Championship, Open Championship, US Open and US Amateur titles.

Dunne, who came through final qualifying at Woburn for the second year running, was in the second group out on Thursday and joked after birdies on the first two holes that he hoped someone had taken a screenshot to prove he had led the Open.

Three days later he had no such worries and could turn his attention from trying to win the silver medal for leading amateur to the Claret Jug.

“I don’t see why not,” said Dunne, who is a lowly 80th in the world amateur rankings. “I mean, I’m well capable of shooting the scores that I need to win if everyone else doesn’t play their best.

“Whether it happens or not, I can’t really control. I can just go out and try to play my game and see where it leaves me at the end of the day. Hopefully I play great again and post a good number.

“It’s surreal I’m leading The Open, but I can easily believe that I shot the three scores that I shot. If we were playing an amateur event here, I wouldn’t be too surprised by the scores I shot. It’s just lucky that it happens to be in the biggest event in the world.

“Hopefully I can do it again tomorrow, but whether I do or not, I’ll survive either way.”

Spieth is looking to become the first player to win the first three majors of the year and just the third to win any three in a single season – Ben Hogan won the Masters and US Open in 1953 but missed the US PGA to compete in, and win, the following week’s Open at Carnoustie, while Tiger Woods won the US Open, Open and US PGA in 2000 and completed the ‘Tiger Slam’ in the 2001 Masters.

“It hasn’t come up in my head while I’ve been playing yet,” said the 21-year-old, who would also replace Rory McIlroy as world No 1 with victory.

“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 that presents itself.

“I don’t look at it as a negative thing, I look at it almost as an advantage. Why should it add more pressure in a negative way?

“If it adds more pressure, it just makes me feel like this is something that’s a little more special, let’s go ahead and get the job done.

“I know it’s easier said than done, but when you say added pressure, most people associate that with negativity or something that can hinder what’s comfortable.

“For me, I think it could be advantageous. You hit the ball a little bit further, you can really get your mind around a more specific target and block out other things.”

American Dustin Johnson, who three-putted the 72nd hole in the US Open last month to finish a shot behind Spieth, found his overnight lead intact when he teed off at 3pm, but struggled to a third round of 75 in ideal conditions.

The big-hitting 31-year-old was only two off the lead when he finally carded his first birdie of the day on the 15th, but bogeyed the last three holes to fall five behind.

“I don’t feel like I played that bad, it’s definitely frustrating,” said the world No 4. “I’m going to have to put together a special round together tomorrow to have a chance.”

Harrington’s lowest round in a major shot him into contention for a third Claret Jug and he believes his experience could be a significant factor on the final day.

“I’ll be certainly telling myself that I know what I’m doing and I’ve done it before and I’ll be using it to help myself, having done it twice before,” said Harrington, whose win at March’s Honda Classic was his first since 2008.