American Curtis dropped shots on the par-five 16th and the par-four 17th to finish with 70, one shot behind South African Els, who hit birdies on the 12th and 16th to finish on 69 at the Bay Hills course in Orlando.
"There's a lot of work left," said Els, who won the WGC-CA Championship earlier this month, ahead of Sunday's final round.
"Obviously, I'm still in a very good position. I would have taken it before the tournament started.
"I would like to keep working and just try and concentrate on playing good golf on Sunday."
American Chris Couch is two shots behind Curtis while Britain's Colin Montgomerie is a further five shots adrift after finishing Saturday on 71 to follow his first and second round scores of 72 and 71.