Bill Haas claimed the lead in the 78th US Masters after carding an opening-round 68 to finish four under par, one shot ahead of defending champion Adam Scott, South African Louis Oosthuizen and and 2012 champion Bubba Watson.

Masters rookies Kevin Stadler, Jonas Blixt and Jimmy Walker are among a six-strong group on two under, alongside Gary Woodland, KJ Choi and Brandt Snedeker.

Rory McIlroy is best of the Irish, three shots off the lead on one under, while 2010 US Open champion Graeme McDowell is another shot further back on level par. Darren Clarke shot two over after recording two back-nine birdies to recover from earlier errors.

Pre-tournament favourite McIlroy safely found the 12th green in regulation but three-putted for a bogey and did the same on the 18th to card 71.

"It was a good day at the office, anything in red figures was a good effort out there," McIlroy said. "When I saw the pin positions this morning I thought they didn't want people to get too many under par.

"They've got the greens very fast already, by Sunday they are going to be pretty dicey. Going out tomorrow afternoon I would not mind if they were a little gentler."

Reigning champion Scott had been four under par after a flawless opening 11 holes, with one of his birdies coming on the par-four 10th where he sealed victory on the second hole of a play-off with Angel Cabrera last year.

But the Australian - aiming to join Jack Nicklaus, Nick Faldo and Tiger Woods in winning back-to-back titles - then dumped his tee shot on the 12th into Rae's Creek in front of the green to run up a double-bogey five, before three-putting from long range for par on the 13th and 15th.

"It was a good day at the office, anything in red figures was a good effort out there" - Rory McIlroy

Three groups ahead of Scott, 50-year-old Miguel Angel Jimenez was also four under after 10 holes in his bid to become the oldest winner of any major championship, Julius Boros being 48 years, four months and 18 days when he won the US PGA Championship in 1968.

However, the Spaniard then dropped a shot on the 11th and also found water off the tee on the 155-yard 12th to card a double bogey on his way to a 71.

Haas had never broken 70 before in 16 previous rounds at Augusta but recovered from a bogey on the first with six birdies, including from five feet on the 18th after also dropping a shot on the 17th.

"I knew about that stat before the round so to shoot in the 60s feels great," Haas said. "There is a lot of golf left but I'm ecstatic with today."