Hillman, a second-year speedster from San Diego State who played sparingly last year, bulked up over the winter and was given first crack at winning the starting job.
He fumbled at the goal line in Seattle a week ago, and it was returned 106 yards for the score. A week later, Rams rookie linebacker Alec Ogletree pried the football from his arms after a short catch and returned it 13 yards for a touchdown.
After that, Hillman didn't touch the ball and the snaps were shared by Ball, who set an NCAA record with 83 touchdowns at Wisconsin, and former first-round pick Knowshon Moreno.
Asked if his promotion Monday was due more to what he was doing right or what Hillman had done wrong, Ball said: "For me, I'm looking at it as what I've done. I've been consistent. Yeah, I've made mistakes, but I've done a great job to not repeat them. That's what I plan to continue to do."
Both Ball and Hillman want to be the featured back in Denver's offense, but with the way Fox has always divvied up carries in his backfield, it might not matter who's the first running back out of the tunnel on Sept. 5 anyway.
"Whoever we keep we're going to lean on and we'll play and I think our track record says that," Fox said.
Ball hasn't been free of blunders, either.
He was penalized for covering up the tight end in a formation against St. Louis and also dropped a pass from Peyton Manning because he said he "smelled the end zone."
A week earlier, he whiffed on blitzing Seahawks linebacker Bobby Wagner, who drove Manning into the ground just as he released a 31-yard pass to Julius Thomas.
"It's the worst feeling as a rookie, seeing a future Hall of Famer get hit like that pretty hard," Ball said. "That's not a good thing at all and I most definitely got chewed out about it."