Defending champion Phil Mickelson and top-ranked Vijay Singh were also in contention, while Tiger Woods and Ernie Els struggled.
The start of the season's first major was delayed nearly 5 1/2 hours by heavy rain, forcing a two-tee start. Even so, only 24 of the 92 players got through the first round, the rest coming back Friday morning to finish.
Not long after everyone had played their first 18 holes, another line of storms made a mess of Augusta National.
The well-worn areas in front of the clubhouse were gooey instead of green. Rae's Creek resembled chocolate milk as it lapped against the 12th green. Workers used squeegees to push away a small creek that formed across the 18th fairway.
The rain cleared out, the grounds crew worked furiously to get the course back in shape, and mud-splattered fans began to reclaim position along the ropes.
But play never resumed. Around 4 p.m., tournament officials called off play for the day with more storms moving in.
Everyone will try again at 8:30 a.m. Saturday.
'It looks very good for the weekend,' said Will Nicholson, chairman of the competition committee. 'But this has been a crazy year.'
Indeed. Bad weather has interrupted play on the PGA Tour for the ninth time in 15 tournaments, and the fourth week in a row. Or course, this is nothing new at Augusta, interrupted by storms for the fourth straight year.
No matter what, the tournament will be played in full - even if stretches into next week. The last Monday finish was in 1983.
'Under any circumstances,' Nicholson said, 'the winner will play 72 holes.'
While waiting around, Howell had lunch with his girlfriend and napped for about an hour in the clubhouse.
'I was right next to Padraig Harrington,' Howell said with a smile. 'He did better than me. He was snoring when I woke up.'
Woods, a three-time Augusta champion, probably welcomed the break.
He knocked a putt in the creek the first day, then had another adventure Friday at No. 8. Woods stuck a shot in the pine trees along the right side, then caught a limb trying to get out. The ball deflected the wrong way but at least settled in the fairway. He wound up with a bogey on the par-5 hole.
Woods finished at 74 and hasn't eclipsed par in the opening round since 2002, when he won the green jacket for the second year in a row and third time overall. He started with a 76 in 2003 and 75 last year.
DiMarco was back in a familiar position - leading the Masters.
He started Friday with a brilliant tee shot at No. 6, a par-3 where he scored a hole-in-one last year. He nearly holed out again, leaving himself a short putt that gave him a stretch of four birdies in six holes on the front nine - albeit over two days.
DiMarco was the clubhouse leader with a 5-under 67, the fourth time he has led a round at Augusta. He likes his chances of being on top when it counts.
'Absolutely,' he said. 'I'm feeling really comfortable and I'm putting well.'
Last year, DiMarco took a lead to the final round before fading with a 76 on Sunday. At least he had a hand in the outcome, providing the line for Mickelson's winning putt at the 18th.
'All I can do is try to put myself into position Sunday and use what I learned last year,' DiMarco said.
Howell, a surprising Ryder Cup star in Europe's dominating win over the Americans last fall, started with a 72. Then, while DiMarco and others were finishing up the first round, he birdied five of his first seven holes in the second to pull even with DiMarco.
'I played lovely,' Howell said.
Donald moved to 5 under with an early birdie in the second round after opening with a 68.
Singh also was one stroke behind DiMarco after the first round, squandering a chance to take the lead at the par-5 15th. He had a 35-footer for eagle, but the putt curled left of the cup. The birdie attempt failed to find the cup again - bitten twice by Augusta's wet but still treacherous greens.
Singh never made it back out for the second round. DiMarco played only five holes all day.
Mickelson was off to a strong start in defense of his title. His first shot Friday dropped 5 feet from the flag at the par-3 12th, leading to a cherished birdie in the heart of Amen Corner. He finished with a 70, just three strokes behind DiMarco.
Els, a perennial Augusta contender, had a steep hill to climb after posting a 75. Of course, he opened with a 79 two years ago and managed to fight back to finish in a tie for sixth.
Donald's 68 included a 20-foot birdie at No. 6. He had a downhill 5-footer to tie for the lead on his final hole, but the ball slid by the cup.
'It's a great start, nothing more than that,' he said. 'There's a long way to go.
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