He overtook James Mason, who had posted a 7-under-par 65 on Friday.
The first round was completed Saturday after lightning forced players off the course for one-hour, 36-minutes Friday leading to the round being suspended due to darkness.
Jim Thorpe, Dave Eichelberger, Stewart Ginn and Jay Sigel all posted rounds of 6-under-par 66 at Hualalai Golf Club.
Tewell capped off a bogey-free round Saturday. He birdied the first and moved to 3 under with an eagle at the fourth. He closed his front nine with birdies at the seventh and the ninth, before birdieing No. 11.
He parred the next two holes before play was suspended. Tewell returned to the course Saturday with a par at the 14th, before a birdie at the next moved him to 7 under and a share of the lead. Tewell then grabbed the outright lead through one round as he rolled home a birdie at the last.
Mason was steady throughout his bogey-free round. He opened by draining a birdie at the par-4 first and came back with his second birdie at the par-5 fourth.
The 53-year-old took advantage of the next par-5, No. 7, as he birdied that to move to 3 under. Around the turn, it was more of the same as he drained his fourth birdie of the day at the par-5 10th.
He made it four-for-four on the par-5s as he birdied the 14th and capped off his round in fine fashion with back-to-back birdies from the 17th.
Dave Barr, Tom Jenkins and Tom Purtzer share seventh place at 5-under-par 67. They were joined there by Fuzzy Zoeller, Gil Morgan and Dana Quigley, the defending champion.
J.C. Snead, Jack Nicklaus and D.A. Weibring are one stroke further back at 4-under-par 68.