A remarkable third round of The Open saw 22-year-old amateur Paul Dunne claim a share of the lead alongside Louis Oosthuizen and Jason Day, as Jordan Spieth significantly enhanced his Grand Slam aspirations.

Spieth's 66 left him one off the pace in fourth at St Andrews and he now has a real chance of becoming only the second man - after Ben Hogan in 1953 - to win the Masters, U.S. Open and Open titles in the same year.

Incredibly, Spieth could yet be denied a shot at an unprecedented calendar Grand Slam by the unheralded Dunne - 80th in the amateur world rankings.

The Irishman displayed admirable levels of composure to raise hopes of a first non-professional success in this tournament since the great Bobby Jones lifted the Claret Jug for a third time in 1930.

A recent graduate from the University of Alabama, who missed the cut on his Open debut last year, Dunne matched Spieth's six-under score with a bogey-free round on Sunday. His 54-hole total of 204 comfortably outstrips the previous record Open mark by an amateur, the 210 set by Iain Pyman in 1993.

Dunne was duly joined at 12 under by Day - a regular major challenger yet to claim a breakthrough success - and Oosthuizen, very much in contention to follow up his victory here in 2010.

Both Day and Oosthuizen shot 67, while two-time Open champion Padraig Harrington moved to 10 under with a 65 and Marc Leishman carded the lowest round of the week - a stunning 64 - to be one of nine players three off the lead, together with the likes of Sergio Garcia, Justin Rose and Adam Scott.

However, benign conditions - in contrast to those that forced the final two rounds to be put back a day - were not capitalised on by the final pairing.

Overnight leader Dustin Johnson could muster only one birdie and bogeyed the final three holes en route to a 75 that left him five behind, while Danny Willett reached 12 under through 10 holes, only to drop back with three dropped shots thereafter.

After finishing his second round five behind Johnson, Spieth suggested he would require "something like 10 under in the last two rounds" to give himself a chance of victory.

And he again demonstrated his ability to respond to setbacks, bouncing back from the day's only bogey at the ninth by tackling the trickier back nine in 32 strokes - thanks largely to three birdies in succession from the 10th.

If Spieth's charge was impressive, Dunne provided an even bigger thrill - taking the outright lead for the first time on the 10th with his fifth birdie of the round and then holding his nerve to mix seven pars with another gain at the 15th.

Day started slowly with four straight pars, but soon clicked into gear and provided a familiar threat, while Oosthuizen birdied three of the last five to make it a three-way tie at the top.

The South African cantered to victory by seven strokes on the Old Course five years ago, but a much tighter battle can be anticipated on this occasion.