Chez Reavie posted an 8-under 64 to join Alex Cejka and Olin Browne in a tie for second at 13 under par.
Lewis, a two-time winner on the PGA Tour, played the Bear's Best layout on Friday, one of three venues in rotation over the first three rounds this week. He parred his first three holes before running off three consecutive birdies starting at the par-3 fourth.
The 44-year-old collected another birdie at the par-5 eighth and kept moving up the leaderboard on the inward half. Lewis tallied a birdie at the 11th followed by an eagle at the par-5 12th. He then birdied the 13th to reach 13 under.
Lewis cruised down the stretch and added a birdie at the par-5 17th to break the logjam and move into first place alone.
'I struck the ball well and knocked in a few putts here and there,' said Lewis. 'I didn't make any mistakes.'
Reavie, who turned pro after this year's U.S. Open, is making his first start on the PGA Tour since he made the cut at Shinnecock Hills and the Arizona State grad is making the most of the opportunity.
'I'm very pleased,' said Reavie. 'It's fun. This is what I dreamed to do.'
Reavie hit a sandwedge to 8 feet for a birdie at the first and picked up another birdie at the par-3 fifth. He then birdied five in a row around the turn to climb to minus-12.
The 22-year-old missed a 10-foot par save for a bogey at the 13th and dropped another shot with a bogey at the 15th. Reavie responded, however, at the par- 3 17th after his tee shot stopped within inches of the hole for a birdie.
Reavie followed that stellar shot with another at the closing hole at the TPC at Summerlin. Standing 161 yards out, Reavie holed a 9-iron for an eagle at the par-4 18th to finish within one of the lead.
'I'm trying not to get ahead of myself, but the goal is winning,' said Reavie. 'This is a dream to be close to the lead. I'm learning to come out here and play with these guys and not be intimidated.'
Cejka, who finished second at The International in August, managed three birdies and a bogey over his first nine holes. He added five birdies on the back side to complete a round of 65 on the TPC at Summerlin.
'I almost had a birdie chance on every hole,' said Cejka. 'I missed a few short ones but I was patient and kept fighting.'
Browne collected nine birdies and a bogey at Bear's Best for a round of 64.
Jay Williamson shot a 9-under 62 at the TPC at The Canyons to finish two shots off the pace.
'The feeling I have right now is I'm just starting on the mountain and I've got a long way to go,' said Williamson. 'It's not a race, it's how many under par you can get.'
Williamson is tied for fifth with Duffy Waldorf, Tim Petrovic, Dean Wilson and Harrison Frazar at 12 under par.
Phil Mickelson, the fourth-ranked player in the world, carded a 66 to finish within five of the lead at 9 under in a group that features Jim Furyk and Justin Leonard.
Defending champion Stuart Appleby posted a 67 to finish in a tie for 51st at 7 under par.