DiMarco, who teed off at the par-four 10th, picked up shots at the first, second and third to get to four under with five holes to play at an overcast Augusta National.
Some of the biggest names in golf were hot on his heels, though, with world number one Vijay Singh three under after 10 holes and defending champion Phil Mickelson two under after 10, level with U.S. Open title-holder Retief Goosen, after 13.
Australia's Mark Hensby, playing his first Masters, held the clubhouse lead on three-under-par 69 after mixing three birdies with a solitary bogey at the par-five eighth.
Also at two under was another Masters debutant, British Ryder Cup player Luke Donald after 13 holes.
However, it was not a day to remember for three-times winner Tiger Woods and world number three Ernie Els.
Woods, chasing his first major title since the 2002 U.S. Open, was two over after 11 holes, having bogeyed the par-five 13th where his eagle putt trickled off the green into Rae's Creek.
PARTIAL RECOVERY
The former world number one, who teed off at the 10th after a 5-1/2 hour delay because of early morning thunderstorms and lightning, partially recovered with a birdie at the par-five 15th.
But he then dropped another shot at the par-four
South African Els, runner-up a stroke behind the triumphant Mickelson at Augusta last year, was one of several players battling away as the gusting winds strengthened.
The three-times major winner bogeyed the 10th, his first hole, the 11th and the 15th and was three over after 11.
Earlier, play was put back five times by organisers due to the persistent threat of lightning before the first of the year's four major championships eventually got under way with a two-tee start early in the afternoon.
However, the weather forecast for the next three days is a lot more optimistic with a possibility of showers on Friday and mainly dry conditions at the weekend.
Should that forecast prove accurate, then round one could well spill over into Friday before the Masters gets back on track for the weekend.
