The third round was played in wet and windy conditions that sent scores soaring as 13 players carded rounds in the 80s.
Eduardo Romero stumbled to a 3-over 73 in round three to fall four strokes behind Roberts at 7-under-par 203. Tim Simpson is alone in third place at 5-under-par 205 after a 2-over 72.
Dick Mast managed an even-par 70, the only other round of par or better on Saturday. He shares fourth place at two-under-par 208 with Gil Morgan (75), David Edwards (76) and Craig Stadler (77). They are the final players under par for the championship.
Roberts tripped to a bogey on the third to fall to minus-9. That was his first bogey of the week, but not his last of the round. He bounced right back with a birdie on No. 4 at the Ailsa Course at Westin Turnberry Resort.
The 2005 JELD-WEN Tradition winner eagled the par-5 seventh to climb to 12 under par. Roberts gave a stroke back with a bogey on the par-4 eighth.
Roberts moved back to minus-12 thanks to a birdie on the par-3 11th. He tripped to the third bogey of his round on the 13th, before carding pars on the final five holes of his round.
'We've got 18 holes to go, so I don't really want to think about it really,' Roberts stated. 'I've had a nice career on the tour, but to win this championship, I just think it would be a feather in my cap. I could feel like I've been fulfilled.'
Romero went bogey-birdie on Nos. 2 and 3, then parred his next five holes to remain at 10 under. Romero bogeyed the ninth, but got that stroke back with a birdie at 11.
The 52-year-old Argentine struggled to back-to-back bogeys from the 14th to slide to 8 under. Romero gave another stroke back as he bogeyed the last to fall four behind Roberts.
'It was a terrible day with the weather,' Romero explained. 'I am second, four behind, but if the weather continues like this tomorrow then four shots is nothing. A 73 is a good score today. I am not angry, but I am not happy. Loren played really good and he is one of the best putters in the world.'
Tommy Nakajima stumbled to a 4-over 74 that dropped him to even-par 210. He is alone in eighth place, while Don Pooley is one stroke further back after a 72 on Saturday.
Peter Jacobsen, who shared the lead after the opening round, faltered to a 7-over 77 to drop to 2-over-par 212. He stands alongside Mark James (75) and Tom Kite (74) in 10th place.