The Atlanta Journal-Constitution reported Deal held about a 9,000-vote lead with 80 percent of precincts reporting. An estimated 475,000 votes were cast, the newspaper said.
Still out were most of the votes from Fulton County, where Atlanta is located, and where Handel is from, the Journal-Constitution said.
"It's too early to know if we're going to win," Deal told his supporters. "There are still some precincts in important areas that are out."
Former Alaska Gov. Sarah Palin, a favorite of the GOP's conservative base, campaigned with Handel. Mike Huckabee, the former Arkansas governor who ran for president in 2008, supported Deal.
The GOP winner will face Democratic former Gov. Roy Barnes in November's general election.