Weir, whose playoff victory in the 2001 season-ending Tour Championship made him the final qualifier for this winners-only event, took advantage of calm conditions at the Kapalua Resort's par-73 Plantation Course. He finished the day without a bogey and three strokes ahead of Frank Lickliter.
'I haven't been on a golf course in a month,' said Weir, who was also brandishing a new set of irons. 'That was way above my expectations.'
Defending champion Jim Furyk shot 6-under 67 to share third place with Scott Verplank, Chris DiMarco, Cameron Beckman and British Open winner David Duval.
Duval, the victor here in 1999, set the course record with a second-round 63 that year.
All eyes were on Tiger Woods, who is looking to avoid the slow start he experienced last year, when he failed to win in his first five appearances. The reigning Masters champion collected two eagles, two birdies and a bogey to join Kenny Perry five shots off the pace with a 5-under 68.
'I wanted to get off to a good, solid start and I was able to do that today,' said Woods, the Mercedes winner in 1997 and 2000. 'I didn't really hurt myself. Even though Weirsy went low, I'm still in the ball game with a good, solid round.'
The two other major winners from 2001 - U.S. Open champ Retief Goosen and PGA Championship winner David Toms - each had 4-under 69s to round out the top 10 with Joel Edwards, Robert Damron and Davis Love III.
Full-field scores from the Mercedes Championships