Torrential rain washed out play on Thursday and Friday as the TPC at Sugarloaf was left unplayable. There were further delays Saturday morning and the tournament was officially reduced to 54 holes with plenty of action set for Monday.
McCarron started on the back nine and picked up a quick birdie at the par-5 10th. He then birdied the par-4 12th and hit a wedge inside 8 feet for a birdie at the par-4 13th. He stumbled to a bogey at the par-5 18th, however, and dropped another shot with a bogey at the second under windy conditions.
The 39-year-old countered with a birdie at the par-4 third after his second shot stopped within 3 feet of the cup. McCarron then reached the green in two at the par-5 fourth and took two putts for another birdie to get to 6 under par with one round to play.
'If the wind dies down, certainly you're going to have to make some birdies tomorrow to compete in this tournament,' said McCarron, whose last victory on the PGA Tour came at this event in 2001. 'It's going to be kind of a horse race out there I think.'
Mickelson won't have much time before the end of this tournament and the start of the Masters but the left-hander is taking everything in stride.
'I was only going to play one practice round on Tuesday, so going into Monday doesn't really affect me much,' said Mickelson. 'I enjoy being able to play a competitive round closer to the start of the Masters.'
Mickelson vaulted out of the gate on Sunday with three birdies over his first four holes and added back-to-back birdies starting at the 10th to move to 3 under.
He then hit his drive within 30 yards of the hole at the par-4 13th and chipped his second shot in for an eagle. Mickelson gave a shot back with a bogey at the par-4 14th but he recovered with a birdie at the last for a round of 65.
'I love playing a week before a major,' said Mickelson, who will defend his title at Augusta. 'I think it's a great place to start hitting the shots that I will need to hit at the Masters.'
Olazabal, a two-time Masters champion, had a difficult front nine with a couple of bogeys, a birdie and a triple bogey. The Spaniard responded on the inward half with two straight birdies from the 10th and an eagle of his own at the par-4 13th.
Olazabal closed in style with birdies on each of his last three holes for a round of 69.
'I chipped it in on 14 and I think that was the turning point on the back nine,' said Olazabal, who had a 29 on the inward side. 'It kept things going my way.'
Mayfair played the back half first and collected a birdie and a bogey over his first nine holes. Mayfair tallied back-to-back birdies from the first to reach 6 under but fell one shot behind McCarron with a bogey at the eighth.
'Overall, I like the way I'm hitting it,' said Mayfair, who carded a 71. 'It's the best I've ever hit it with the way the wind has been blowing and all that. I've just got to be patient tomorrow and hopefully get some putts to drop early, and who knows.'
Defending champion Zach Johnson fired a 6-under 66 to finish at 3-under-par 141 alongside two-time U.S. Open winner Retief Goosen, Ryuji Imada, Brandt Jobe, Bob Tway, J.L. Lewis and Frank Lickliter.
The second round will resume at 8:00 a.m. ET on Monday with the final round scheduled to follow.