TEMPE, Ariz. -- Arizona State's offense stagnated, its defense was on its heels.
A blocked field goal turned the momentum back on the Sun Devils' side and Robert Nelson sealed it with another big play in the fourth quarter.
Next up: A showdown with No. 13 UCLA with the Pac-12 South title on the line.
Nelson returned his second interception 23 yards for a touchdown in the fourth quarter and No. 21 Arizona State maintained control of the Pac-12 South with a 30-17 win over Oregon State on Saturday night.
"Everything we've worked for is in the 60 minutes of this next game," Arizona State coach Todd Graham said. "I ain't going to lie to them."
Arizona State (8-2, 6-1 Pac-12) dominated the first half, going up 20-0 on two touchdown runs by Marion Grice, who became the third Sun Devil to score 20 touchdowns in a season after running for 118 yards.
The Sun Devils stumbled through the third quarter, though, struggling on offense while allowing the Beavers to march down the field.
Arizona State halted Oregon State's momentum when Davon Colemen blocked a field goal in the fourth quarter and Nelson, who has a knack for rising up in big moments, put it out of reach by racing into the end zone on Sean Mannion's fourth interception of the game.
The Sun Devils beat Oregon State for the 19th time in 20 games at Sun Devil Stadium and have won five straight games overall, setting up next week's showdown with the Bruins in Southern California.
"It's huge for our program," said Arizona State quarterback Taylor Kelly, who threw for 183 yards on 22-of-37 passing. "Our guys believe in each other. Whether we're having problems on offense, defense or special teams, we come together as a group."
Oregon State (6-4, 4-3) got off to a difficult start and tried to rally in the third quarter, only to come up short for its third straight loss.
Brandin Cooks had nine catches for 99 yards and has 100 receptions on the season to break the school record of 91 held by James Rodgers and Markus Wheaton.
Mannion threw for 320 yards and two touchdowns, but has seven of his 10 interceptions this season in the past two games.
"I am disappointed in the way I played," Mannion said. "I take a lot of responsibility for this loss. I expect more from myself."
Arizona State needed this game to preserve its lead in the Pac-12 South with the Bruins right on its heels.
Oregon State needed it get back on track after following a six-game winning streak with losses to Stanford and USC.
The Sun Devils dominated at the start.
On offense, they raced 75 yards on their opening drive for a touchdown run by Grice, his first in three games. Jaelen Strong had 66 yards receiving on the drive, 16 more than the previous three games combined.
Arizona State was derailed briefly when Kelly threw interceptions on the next two drives -- one due to pressure, another on an underthrow -- but kept rolling after that.
D.J. Foster scored on an 8-yard run -- the extra point was blocked -- and Grice followed a muffed punt by Brandin Cooks with another TD run to make it 20-0. Grice joins Terry Battle (20 in 1996) and Wilford White (22 in 1950) as the only Arizona State players to score 20 touchdowns in a season.
Arizona State's defense was even better, dominating the nation's 25th-best offense for most of the first half.
The Sun Devils held Oregon State to 38 total yards in the first quarter and intercepted Mannion twice in the half. Oregon State finally got a sustained drive going in the closing minutes, but had to settle for Trevor Romaine's 22-yard field goal at the horn after Arizona State's Alden Darby tackled Micah Hatfield at the 5-yard line with a second left.
After opening with an interception -- Darby had to dive to get it -- Mannion got the Beavers rolling again, picking apart the Sun Devils on a 96-yard drive capped by Caleb Smith's 6-yard touchdown catch.
Oregon State held Arizona State to 34 yards in the third quarter and its offense was moving again in the fourth, but center Isaac Seumalo was called for an unnecessary roughness penalty to back the Beavers up. Coleman then got a hand up to block Romaine's 48-yard field goal attempt and Arizona State got back to its quick-striking ways on the ensuing drive, going up 23-10 on Zane Gonzalez's 18-yard field goal.
Nelson sealed it with his first career pick-six, stepping in front of Mannion's intended receiver and racing into the end zone.
"We started slow," Oregon State linebacker D.J. Alexander said. "We let them get up on us and we tried to react, but it was too late."