Tiger Woods shot his best round of the year to claim a share of the lead at the Arnold Palmer Invitational at Bay Hill in Orlando but England's Paul Casey also shot 64 to tie the world number one.
Vaughn Taylor was the third person to card a six-under-par 64 but all the attention was on Woods and Casey.
Darren Clarke carded a disappointing three-over-par 73 in his opening round.
Woods has not played much in 2007 but when he claims he has just produced his best performance of the year, it still means something.
'It is definitely the best round of the year (although) I haven't played that many,' said Woods, who won this event four successive years from 2000.
'It was fun to hit the ball as well as I did and, more important, get off to a quick start, especially with the [forecasted stormy] weather coming.
'I controlled my flight all day, even when the wind started picking up. I missed a couple of putts early but after that I rolled the ball really well.'
Woods drove the ball well, avoiding the punishing rough on all but a couple of occasions.
He missed only one green in regulation, at the par-four 18th, his ninth hole, where he had little choice but to take a penalty stroke after his second shot ended up in the rocks guarding the green.
Casey did something Woods could not manage, a bogey-free round, with the 29-year-old capping off his day with a seven-foot birdie at the final hole.
'It was the best start to a tournament I've had this year," Casey said.
'I got a little stale in the middle and then finished it off. To birdie 18 was fantastic, and I'm ecstatic with that.
'The greens were very receptive. I think that was the key [to the low scores]. If you have a good lie in the fairway and the right club in your hand you can really attack the flag.
'I had a lot of clubs which were right on the number, and allowed me to go straight at the flag.'
Casey said it was his best round at Bay Hill, although in his only previous appearance he pulled out after an opening 77 .
The Englishman has struggled on the green in his past few tournaments but some hard work last week with coach Peter Kostis seems to have paid off.
'The posture got a little slumped, shoulders rounded,' he continued. 'I wasn't in a good position to hit putts, so we worked very hard on the posture and tried to build up the confidence last week, and it seems to have worked very nicely.'
Despite his world ranking of 13 Casey, by his own admission, has been an underachiever on the PGA Tour, where he has never finished better than fourth.
'It's without doubt one of my top goals [to win in the United States], and if it was an event where we have a stellar field like this, the world's best and also Mr Palmer's event, that would make it doubly special, he added.
Two strokes off the lead is a group which contains Casey's Ryder Cup team-mate Sergio Garcia, Carl Pettersson, Trevor Immelman, Rocco Mediate and Sean O'Hair.
Eleven players did not complete the round due to a late afternoon storm and will have to return today.
