'Ninety percent of it is carryover from Sunday,' admitted Montgomerie, who finished second at this year's British Open. 'If I finished third or fourth on Sunday I wouldn't have shot 64 today. Simple as that.'
Montgomerie has built a three-shot lead after round one, but no fewer than 11 players are right behind him, including the top-two players in the world rankings.
Tiger Woods and Vijay Singh headline the group in second place at 3-under-par 67. Sergio Garcia, Chad Campbell, Rod Pampling, John Daly, David Howell, Ian Poulter, Deutsche Bank Championship winner Olin Browne, Canadian Open titlist Mark Calcavecchia and Players champion Fred Funk joined Woods and Singh.
Woods only netted two birdies on his front nine, but got to 3 under for the championship with a 6-foot birdie putt at the first hole. He reached 4 under with a birdie two holes later, but things fell apart down the stretch.
He was not able to give himself many good looks at birdie on his second nine and missed a short par putt at the eighth. Woods parred his last and is in position for his fourth win at this event.
'If I shot something between two- and 4 under par, I thought it would be pretty good,' said Woods, who had several of his former Stanford Cardinal teammates in the gallery on Thursday. 'Especially looking at the pin locations and how tucked they were, I thought that would be a pretty good score.'
Singh collected three birdies over his first 12 holes, then could not buy another birdie. He was in trouble at the very difficult par-4 ninth, but holed a nice par save to stay in a tie for second.
'You can score very low here. Well, you think you can score very low here, but the greens are so hard to read. They're really tough,' said Singh.
Both players, and the remainder of the field, are staring up at Monty, who posted a bogey-free round on Thursday.
Montgomerie began on the back nine at Harding Park on Thursday and promptly birdied the par-5 10th from 4 feet. He rolled in a 25-footer for birdie at the 12th hole, but ascended into first with his play at the end of his opening nine.
The Scotsman closed his first nine with three birdies in a row, all from inside a few feet. Montgomerie lipped out birdie putts at three and four, then held on to the lead when one of his Ryder Cup teammates charged up the leaderboard.
Montgomerie drained a 10-footer for par at the sixth to hold off Garcia, one of his playing partners. The Spaniard birdied six to get to 4 under, and Montgomerie sank that clutch par save to stay one ahead.
The short, par-4 seventh was a big hole for Montgomerie as Garcia once again looked to be in great shape for a birdie as his approach landed 3 feet from the stick. Monty hit his second to a foot for the guaranteed birdie, then watched Garcia miss, giving him the clear advantage after only 18 holes.
'I hit most of the fairways and if I hit the fairways I can hit the greens and then I can score,' said Montgomerie. 'The round was made on 16, 17, 18. Especially 18, that's the toughest hole on the course, and it was nice to birdie that and it got me really going at 5 under.'
Montgomerie has had a Hall of Fame career capped by seven consecutive Order of Merit titles and seven straight Ryder Cups. There are two glaring omissions on his golf resume - a major championship and an official win in the United States.
'There's no secret that I would love to win an event over in America before I finish,' said Montgomerie. 'Whether that happens or not, it won't change my life either way. I'm not suddenly going to change my lifestyle because I've won, but I would love to win over here.'
David Toms, Jim Furyk and Zach Johnson, who will be representing the United States in the WGC-World Cup with Stewart Cink, are part of a group tied for 13th at minus-2.
Reigning PGA champion Phil Mickelson shot a 1-over 71 and is tied for 36th place.
Ernie Els, the 2004 winner, is not defending because of his knee injury and fellow countryman Retief Goosen is also out this week because of a groin injury.