Janzen, trying not to be too aggressive in a tournament where birdies are made in bunches, had 10 birdies and one bogey and was tied at 9-under with Steve Jones.
Jones had a 62, but his came at the par-71 TPC Canyons course, while Janzen played the par-72 TPC Summerlin course.
Both were a stroke ahead of PGA champion Rich Beem and J.L. Lewis.
'It's easy to press because you think you have to shoot 63 every day,' Janzen said. 'I make more birdies now that I play more conservative if you can believe that. If I just stay away from gambling on too many holes I do well.'
On a warm and windless day made for scoring, Janzen finished off a back-nine 30 with a 5-footer for birdie to get to the top of a scoreboard crowded with red numbers.
It might have been even better if not for a few missed short putts, including a 2-footer for birdie on No. 9 that didn't go in. Janzen promptly followed that with birdies on 10 and 11 to get his round going again.
'I didn't want to let that ruin my round,' Janzen said.
Winless since winning his second U.S. Open in 1998, Janzen said he is hitting the ball better than ever but just hasn't been able to make enough birdies when it counts to win.
Janzen won seven times on the PGA Tour through 1995, but only once since.
'In 1995 if someone had said you're going to win only once more over the next seven years I wouldn't have believed it,' Janzen said. 'It's just a matter of getting the right attitude back on the course to win again. I think I'm headed back in that direction.'
If Janzen is to win here, he'll have to have four more days of scores well under par. His 63 came on a course that played four strokes under par to the field and didn't give him much breathing room.
Jones, like Janzen a U.S. Open champion, had 11 1-putts for the 62 that tied his lowest round ever. Jones made nine birdies and no bogeys.
'If you're making pars out here you better go home,' Jones said.
Beem used the same 7-wood he made eagle with to beat Tiger Woods in the PGA Championship for another eagle in his 64.
Beem hit the 7-wood to 20 feet on the par-5 13th for the first of two back-nine eagles. He finished his round by pitching in from 67 yards for another eagle.
The stakes weren't quite as high as the 7-wood he hit for an eagle on No. 11 that final day to win the PGA Championship, but Beem was just as pleased with both the shot and the club.
'That's one of the few clubs that will never come out of the bag,' Beem said.
Looking for a third win to cap off a spectacular breakthrough year, Beem finished off his round with a flourish by pitching in from 67 yards for an eagle on 18 at Southern Highlands Country Club.
Beem, playing in his fourth event since winning the PGA, is fifth on the money list with $2.83 million. He would add another $900,000 to that if he can keep up his play over a marathon tournament that goes 90 holes over three courses.
Beem's best finish since winning the International and then following it with the PGA was a sixth in the NEC Invitational. He admitted having some trouble adjusting to the demands placed on a major championship winner.
'I'm still a little tired from everything but slowly and surely I'm catching up on my sleep,' he said.
Lewis, whose only win came in the 1999 John Deere Classic, shot his 64 at the TPC Summerlin course, where the final two weekend rounds will be held.
The 64 may have looked impressive, but Lewis said the course was playing easy because all the par-5s were reachable in two and the greens were nearly perfect.
'Average is about 4 under here if the wind doesn't blow,' Lewis said. 'You should be able to get it around in that if you don't make any big mistakes.'
Chris Riley, who played at UNLV and lives on the TPC Summerlin course, was in contention after a 65 that included a 29 on his back nine.
Riley, who contended in the PGA Championship before winning the next week at the Reno-Tahoe Open, had a 65 after opening with an even-par 36 on his front nine.
Riley said he had problems focusing on the front nine in one of the few events where amateurs play with the pros.
'I just find myself being too nice,' Riley said. 'It took me until about the seventh hole to get focused.'
Full-field scores from the Invensys Classic at Las Vegas