Leonard, who won the 2000 Westin Texas Open to qualify for this week's event, fired a 6-under-par 67 on the par-73 Plantation Course in Kapalua, Maui, Hawaii to take a one-shot lead over last year's runner-up Ernie Els. Defending champion Woods is three off the pace after opening in 3-under-par 70.
Five players are tied for third place. Billy Andrade, Rory Sabbatini, Michael Clark II, Stewart Cink and Jim Furyk each carded rounds of 4-under-par 69 on Thursday.
If Tiger should fall to Leonard this week, he can put some of the blame on his teacher, Butch Harmon. Leonard sought out the famed instructor's help at the end of last year, and it seems to have worked early in 2001.
Never one of the longer hitters on Tour, Leonard has shifted the ball further up in his stance to rectify some timing issues.
Leonard is one of 18 contestants in the 33-man field who played in last week's WGC-Accenture Match Play in Melbourne, Australia. The 28-year-old Texan won a pair of matches before falling to eventual champion Steve Stricker in the third round.
Thursday, Leonard posted eight birdies to just two bogeys in above average windy conditions. Along the way, the '97 British Open champion hit all 15 fairways and 17 of 18 greens in regulation.
'The holes that I'm supposed to score low on, I feel like I did, with little wedges and things like that,' Leonard said. 'I played pretty well through the elements.'
Stagnate throughout most of the round, Els made his climb up the leaderboard by playing the final four holes in three-under-par. After birdying the par-5 15th, the South African eagled the 663-yard par-5 finishing hole.
Els, who eagled the 18th in the final round a year ago to force a playoff with Woods, drove his ball a massive 417 yards on the downhill, downwind hole. Left with 246 yards to the hole, Els smoothed a five-iron to within eight feet of the cup, which he then converted for an eagle-three, and a 5-under-par total.
'The scoring holes on the front nine, I didn't play very well,' said Els. 'But I played a good back nine. Sixty-eight, gotta take it. It's a good start.'
It took Tiger five holes to record his first birdie of the 2001 season; he did so by two-putting the par-5 5th. Woods then drove the 398-yard par-4 6th, which led to another two-putt birdie.
In position off the tee to birdie the par-5 9th, Woods played a driver off the deck for his second shot and found nothing but roughage. Tiger played a provisional to the front of the green, yet the shot proved to be superfluous, as Tiger's playing companion, Paul Azinger, found Woods' original ball in the waist-high weeds.
Forced to hack out his third shot, Tiger could do no better than chop a wedge into a fairway bunker. He eventually bogeyed the hole, which proved to be the lone blemish on his card.
'I probably shot the worst score I could have shot today,' said Woods, who played the back nine in 2-under-par 35. 'But nonetheless, I'm in the tournament.
'I'm in good shape right now. I hope I can get it going tomorrow.'
MORE on The Mercedes Championships:
Justin Leonard First Round Interview Transcript
Ernie Els First Round Interview Transcript
Tiger Woods Pre-Tournament Interview Transcript