Smyth, a first-round co-leader, posted a 5-under 67, while Gary McCord, who shared the lead on Friday with Smyth, only managed a 3-under 69 for sole possession of third at minus-10.
Tewell did not get off to a great start on Saturday and he even found trouble before he teed off. His left elbow gave him problems on the putting green and he could not get any medical attention. On the first fairway, Tewell was given an air cast, but he bogeyed the hole.
'The first hole was not very pretty,' said Tewell. 'I got the cast in the middle of the fairway and I stuck it on my arm. It still hurts, but it's working so I'm not going to mess with it.'
Tewell rebounded with birdies at the second and fourth to get back it, but his play at the end of the front nine is what got him into first. Tewell made a 12-foot birdie putt at the seventh, then collected back-to-back birdies when he holed a 9-footer at the eighth. He tallied his third birdie in a row at No. 9 to make the turn at 4-under 32.
Things stayed hot for Tewell on the second nine. He parred the 10th, but left himself with 20 feet for eagle at the par-5 11th. Tewell missed the eagle putt, but tapped in for birdie. At the par-3 12th, Tewell stuck his tee ball inside 2 feet and kicked in his birdie putt.
Tewell missed the green at the par-4 15th, but got up-and-down for par. His tee ball at the par-3 16th came up 60 feet short and Tewell ran his birdie putt 8 feet past the hole. Tewell converted that par save as well to keep the lead.
At the 17th, Tewell knocked his second shot to 17 feet. The 55-year-old sank the birdie putt and looked to extend his lead at the TPC at River's Bend's closing hole.
Tewell laid up short of the putting surface with his second shot as the hole could not be reached thanks to the wind. He hit a solid wedge shot 8 feet beyond the hole and his putt did everything, but fall into the hole.
Tewell settled for par and his first second-round lead since the season-opening MasterCard Championship, where he tied for third.
'You had to keep going low,' said Tewell. 'If I get the putter working, that seems to be my Achilles heel. I felt good about the putts I hit all day. I hit a good putt at 18. I really thought I made it.'
Tewell picked up his first win of the season a few weeks back at the Greater Hickory Classic at Rock Barn thanks to a Sunday 64. Since the trip to the winner's circle, Tewell tied for fourth place at the Tradition and shared ninth last week at Pebble Beach.
'I'm very comfortable with my game right now,' said Tewell, an eight-time winner on the elder circuit. 'I feel very confident. I'm excited to get out there tomorrow and get after it.'
Smyth, a two-time Ryder Cupper from Ireland, recorded birdies at the fourth and sixth holes. He had a 20-foot look at eagle at the eight, but came up 3 feet short. Smyth converted the birdie putt to hang with Tewell near the top of the leaderboard.
Smyth made five consecutive pars around the turn before a birdie at the 14th. He was two down to Tewell when Smyth played 17, and Smyth followed Tewell in with birdie.
He missed his first fairway at 18 and laid up in a fairway bunker. Smyth's pitching-wedge approach from the sand hit the green, but spun back to the fringe. He two-putted for par as he looks for Champions Tour victory No. 1.
Fred Gibson fired a 65 on Saturday and is alone in fourth place at minus-7. Defending champion Gil Morgan (67), Hale Irwin (67), Jim Thorpe (68) and Bob Murphy (69) are knotted in fifth place at 8-under-par 136.