BOULDER, Colo. -- The Colorado Buffaloes scored 10 points in the final 24 seconds of the first half and talked at the break about pinning the Arizona State Sun Devils deep to start the third quarter.

Rashad Ross ruined those plans.

He returned the second-half kickoff 100 yards for a touchdown that turned a close game into a 51-17 rout of the Buffaloes on Thursday night.

It was just his third kickoff return of the season.

"So when I went back there I was like, 'Man, I've got to take this to the house because this might be my only opportunity,'" Ross recounted. "From there, mentally I was so ready, so as soon as I got the ball, no matter what I was going to run fast and find a hole."

He did just that.

"When I got to full speed, I saw a hole, and when I went through the hole I was like, 'I'm going now,'" Ross said. "No one got a hand on me."

And the Buffaloes couldn't catch the Sun Devils over the next 29:49, either.

"That was a big turnaround for us," Sun Devils safety Chris Young said. "He got some momentum for us. That helped out a lot."

Taylor Kelly threw for 308 yards and five touchdowns, including three to tailback Marion Grice covering 37, 16 and 20 yards as the Sun Devils (5-1, 3-0 Pac-12) won their third straight game heading into their showdown next week against second-ranked Oregon.

Kelly was 20-of-28, and when he wasn't completing screen passes, he was tucking the ball and taking off for equally big gains, finishing with 67 yards on scrambles.

The Buffaloes (1-5, 1-2) fell to 2-1 at home in Thursday night ESPN games. They beat Stanford in 1990 and West Virginia in 2008 and trailed 20-17 in this one at the break after scoring 10 points in the final 24 seconds of the first half.

At halftime, Buffs coach Jon Embree said, "We have a lot of momentum."

Not for long.

Ross took the kickoff a yard deep in his end zone and darted down the right sideline untouched, reaching the other end zone 11 seconds later to make it 27-17.

"We talked about coming out of the locker room doing a great job covering the kicks," Embree said. "And, obviously, we didn't do that."

Ross's return kick-started a 31-0 second-half onslaught by the Sun Devils, who admitted they might have been peeking toward the Ducks next week before getting their heads straight at halftime and watching Ross turn the tide.

"It did deflate us," Buffs defensive tackle Will Pericak said. "It's tough to see one play like that turn a game."

"It is tough," Buffs quarterback Jordan Webb said. "We went into the halftime with all the momentum, we were really confident coming out. For that to happen really took the air out of the stadium."

Just before halftime, Webb hit tight end Nick Kasa over the middle for a 23-yard TD, and Jamal Miles fumbled the ensuing kickoff at his 17. K.T. Tu'umalo recovered for Colorado and after two shots into the end zone came up empty, Will Oliver nailed a 36-yard field goal as the first half expired.

The Sun Devils struck first when Kelly threw a screen pass to Grice, who weaved his way for a 37-yard score in the first quarter. Nate Bonsu blocked Joe Mora's extra point, leaving Arizona State with a 6-0 lead.

"That's just one less point that they have," Bonsu said at halftime. "And that's huge because that could be the difference in the game."

The Buffs couldn't keep up with the up-tempo Sun Devils to make that miss matter, however.

Colorado took a 7-6 lead on Tony Jones' 2-yard run, but three minutes later they lost that lead and never got it back.

Alex Garoutte replaced Mora after the Sun Devils' next score and kicked his 25th PAT in 25 tries, giving Arizona State a 13-7 advantage following Kelly's 34-yard strike to running back D.J. Foster.

Will Sutton's 8-yard sack of Webb helped the Sun Devils force a three-and-out and the Arizona State offense needed just three plays and 61 seconds to stretch its lead to 20-7.

Keller hit Chris Coyle for gains of 30 and 18 yards, then threw a frozen rope 16-yard touchdown pass to Grice, the ball zipping just past linebacker Jon Major's helmet as he failed to turn his head.

Mora came back to kick a 38-yard field goal in the fourth quarter that made it 37-17, then kicked the extra point following Richard Smith's 31-yard touchdown catch.

The Sun Devils capped their second-half onslaught with Cameron Marshall's 14-yard TD run with 3:34 left following Robert Nelson's interception.

That score came a couple of minutes after Buffs linebacker Brady Daigh was strapped to a backboard after injuring his neck on a tackle of backup quarterback Michael Eubank with 4:40 left in the game. Daigh was conscious and moving but was transported to Boulder Community Hospital for precautionary medical tests.

"Right now, things look OK," Embree said.

Colorado was already without starting tailback Christian Powell (thigh) and three defensive starters: free safety Ray Polk (ankle), linebacker Paul Vigo (foot) and cornerback Yuri Wright (concussion).

"We might have overlooked Colorado just a little bit," Young said. "We came out with the W. We played a great second half."