The duo is one shot clear of Terry Mauney, Howard Twitty, Tom Kite, Joe Inman and Tom Wargo ' all of who fired 5-under-par 67s.
Neither Rachels nor Doyle have won this season; though, Doyle has 12 top-10 finishes in 15 starts.
Rachels is in his first season on the Senior PGA Tour. The 50-year-old Floridian won the 2000 Southern PGA Club Professional Championship. He earned full-exemption onto the tour by finishing sixth in the National Qualifying Tournament.
Despite back problems, Rachels played on the PGA Tour from 1975 to 1985 and had 11 top-10 finishes in 123-career starts.
Thursday, Rachels recorded eight birdies and two bogeys to tie his lowest round of the season.
Full-field scores from the BellSouth Senior Classic