The build-up to the tournament for the world number two had been fraught with questions and concerns over his patchy form so far this year but the American answered emphatically, shooting a 66 which included six birdies.

The two-time major winner hit three in the first nine holes and repeated the feat after the turn to enter the clubhouse as the outright leader.

New Zealander Danny Lee and Ireland's Shane Lowry are the cloest challengers to Spieth after both carded four-under rounds of 68.

Five players - English trio Paul Casey, Justin Rose and Ian Poulter, Soren Kjeldsen of Denmark and Spain's Sergio Garcia - are all well placed on three-under.

Northern Irishman Rory McIlroy, who will become only the sixth man to clinch the career Grand Slam if he prevails on Sunday (April 10), had looked as though he was about to mount a serious assault on Spieth's lead but bogeyed the last to end on two-under.

Spieth’s advantage was under threat from new world number one Jason Day of Australia, who came into the event off the back of triumphs at the Arnold Palmer Invitational and the WGC-Dell Match Play, before he endured a disastrous back nine, dropping five shots in three holes on his way to finishing level par.

"I got the most I could probably get out of my round today," Spieth said.

"I think it may have been better than the first round last year because of the conditions - I definitely could make that argument.

"I'm very pleased with it and I put it up there with one of the best rounds I've played; one of the best rounds I've scored."

It proved to be a day to forget for four-time major winner Ernie Els, who recorded the worst hole in Masters history when he took nine strokes to complete the first.

The second round begins tomorrow at 8:20am local time.