DiMarco, a perennial contender at Augusta National, shot his second straight 67 for a comfortable lead as the weather-delayed tournament finally reached its midway point Saturday afternoon.
DiMarco had a 10-under-par 134, good enough for a four-stroke lead over Denmark's Thomas Bjorn and six ahead of three-time winner Tiger Woods.
Top-ranked Vijay Singh and England's David Howell, a surprising star of last fall's Ryder Cup, were at 141. Defending champion Phil Mickelson was in the group another stroke back -- one day after a nasty confrontation with Singh over spike marks.
'Chris is playing well,' Woods said. 'Obviously, he's doing all the things right. Eleven birdies is not too bad.'
Nicklaus wasn't anywhere close to the lead. Afterward, he said he's done at Augusta -- for good.
After shooting a 9-over 153 and missing the cut by five strokes, the six-time Masters champion said he won't be back to play again.
The 65-year-old Nicklaus wiped away tears as he strolled up the ninth fairway, his final hole. He came back for this Masters at the urging of club chairman Hootie Johnson, just a few weeks after the drowning death of Nicklaus' 17-month-old grandson.
'I knew it was my last time walking up the fairway,' the Golden Bear said. 'Obviously, I had made up my mind. This is just too tough for me. I just can't do this.'
As he walked off the green, Nicklaus tipped his cap to the cheering gallery, stood for a few seconds to take it all in, then handed the ball to his caddie and son, Jackie.
'This was a treasure for me,' the Golden Bear said. 'I'll miss that -- greatly.'
DiMarco is leading a round at the Masters for the second straight day and fifth time in five years, but he still must prove he can be on top when it counts.
'I noticed how I separated myself from the field,' he said before starting the third round. 'I'm glad to be able to go back out today.'
DiMarco also had a 36-hole lead in his Augusta debut in 2001, and he was on top going into the final round a year ago before fading to a 76 in the final group with Mickelson.
Asked the last time he felt this good at the Masters, he replied, 'This time last year.'
DiMarco built his big lead on the par 5s. Over the first two rounds, he posted six birdies in eight tries on the long holes, and played solid everywhere else. Through 36 holes, he had only one bogey.
Bjorn had two eagles on the back side and birdied his final two holes, denying DiMarco the largest 36-hole lead in Masters history. The record remains at five strokes, by Herman Kaiser in 1946, Nicklaus in '75 and Raymond Floyd in '76. All three went on to win the tournament.
Woods came to life after a shaky first round. He had seven birdies on his way to a 66 -- the best round of the tournament so far and just one stroke off his lowest Masters score. He shot 65 in the third round of the 1997 Masters, on the way to a record 12-stroke victory.
'You've just got to keep plugging along,' Woods said.
He opened with a 74, even knocking a putt into Rae's Creek. It was the third straight year Woods has failed to break par in the first round of the Masters.
No one was able to complete the second round on Friday. Rain swept across Augusta early in the afternoon, wiping out play for the rest of the day.
The golfers returned Saturday morning to a thick blanket of clouds and a stiff breeze. But the sun finally broke through shortly after noon, warming things up and helping dry out the soggy course.
The forecast for Sunday was promising as well -- sunny, with temperatures in the upper 70s.
On Friday, Singh complained to rules officials that the metal spikes in Mickelson's shoes were too long and creating marks on the green.
Playing in the group behind Mickelson, Singh raised the issue at No. 12 after missing a 25-foot birdie attempt on about the same line that Lefty used to make his putt.
Mickelson wasn't happy about the way the situation was handled, and he said so to Singh afterward in the clubhouse -- a rare outburst in the genteel sport.
'I heard Vijay talking to other players about it, and I confronted him,' Mickelson said through his press agent, T.R. Reinman. 'He expressed his concerns. I expressed my disappointment in the way it was handled. I believe everything is fine now.'
Singh always seems to be in contention no matter where he plays. Even with the spike brouhaha, he held himself together and was in contention for his second green jacket.
Mickelson was trying to become just the fourth player to win the Masters two years in a row. Nicklaus, Woods and Nick Faldo are the only repeat champions.
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