ST ANDREWS, Scotland - Paul Dunne went around the Old Course at St Andrews in record-breaking form Sunday and launched himself into a share of the third-round lead in the 2015 British Open.

Dunne, a 22-year-old amateur from Dublin, shot a third-round score of 6-under-par 66 to set a new amateur scoring record for British Open third rounds, and his 54-hole total of 204, (12 under) smashed the Open's amateur 54-hole record by six shots -- the old record of 210 had stood for 22 years.

Dunne shares the lead with Jason Day and Louis Oosthuizen, the 2010 British Open champion.

"It's surreal I'm leading the Open," said Dunne, a graduate of the University of Alabama-Birmingham who finished fifth at the NCAA championships in May. "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."

The last amateur to win the British Open was Bobby Jones at Royal Liverpool in 1930. It would be remarkable for Dunne, whose caddie this week is his UAB coach, Alan Murray, to hold his form and nerve and defeat a world-class professional field to lift the Claret Jug. He also is vying for the Open's silver medal for the leading amateur, and Dunne has a three-shot lead over Oklahoma State's Jordan Niebrugge, 21, in that race.

"To go out there and be leading the tournament, you can't take that away from him ever," said fellow Dubliner Padraig Harrington, the British Open champion in 2007 and 2008 who sits two shots off the lead. "He's led the Open as an amateur. That is as rare as it comes. If I don't win, I hope he does and it is certainly not beyond belief to see him continue and win it from here."

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Jordan Spieth, twice a U.S. Junior Amateur champion, also said he was not surprised to see an amateur at the top of the leader board.

"There will be an amateur who wins a PGA Tour event, possibly even a major, I think, at some point over the next decade or so," said Spieth, who is a shot back of the lead as he attempts to win his third straight major. "The amateur game has changed to be more like professional golf in that there are more tournaments, better golf courses, harder courses and better competition. That's how I felt playing amateur golf. There's just no fear and I think it's awesome.

"We certainly expect [Dunne and Niebrugge] to make a run [Monday] and I don't expect them to really fall off at all."

Wearing the same Under Armour cap and clothing line as Spieth, and with a similar build, Dunne has endured fans at the Open mistaking him for Spieth, only for him to look up and see disappointed faces when they realize the mistake. If he can emulate Jones and win the Open, Dunne won't need to worry about mistaken identity any more.