It's not often that a coach will rave about a player who failed to score a touchdown -- on purpose.

But USC wide receiver JuJu Smith-Schuster, with nothing but green grass ahead of him, went down to the turf with less than two minutes left, allowing the Trojans to safely run out the clock and beat No. 21 Colorado 21-17 in a Pac-12 game at the Los Angeles Coliseum on Saturday.

"Unbelievable," USC coach Clay Helton said. "A great play by a veteran player to get the first, know situational football, get down and win the game."

Smith-Schuster could have scored on that third-and-7 reception, but settled for a 25-yard gain to the 14-yard line.

The Buffaloes (4-2, 2-1) were playing as a ranked team for the first time since 2005.

USC (3-3, 2-2) was in control for much of the game but committed four turnovers and needed to answer after Colorado tied the score at 14 with 10:20 left on a 10-yard pass from redshirt freshman quarterback Steven Montez to wide receiver Bryce Bobo.

The Trojans responded with a 70-yard drive, capped by a 7-yard pass from quarterback Sam Darnold to tight end Tyler Petite, his second scoring reception of the day.

Colorado converted a 42-yard field goal by freshman Davis Price with 4:49 left but never got the ball back.

"We had opportunities to go take the game and we didn't do it," Buffaloes coach Mike MacIntyre said.

"I will say this: Our young men kept fighting and fighting and fighting and fighting. We had an opportunity to win the game and we didn't get it done. I think we're a good team and they're a good team."

Sam Darnold threw three touchdown passes to offset his three turnovers. Darnold, a redshirt freshman making his third start, completed 25 of 37 passes for 358 yards.

"If we didn't turn the ball over, I think the score is a lot different," said Darnold, who lost a fumble into the end zone on USC's first possession.

"But I think Colorado forced those turnovers on us. They did a great job stripping the ball. Very well-disciplined team, and that is a credit to coach MacIntyre. I feel a little lucky to come out with this win."

The Trojans led 14-0 at halftime but gave away the ball three times in about the first five minutes of the second half, including another fumble by Darnold, who coughed up the ball at the Colorado 33 as he was being sacked.

One play later, Colorado scored on a trick play. Bobo (career-high 10 catches for 83 yards) took a lateral to the left and threw deep to the right sideline for running back Phillip Lindsay, who scored on a 67-yard reception.

"I think in the first half they pushed us around a little bit, but then we gathered our momentum and saw what they were trying to do to us," MacIntyre said. "It gave us an opportunity to make some plays in the second half."

Darnold was picked off on the ensuing possession by linebacker Kenneth Olugbode, but Trojans cornerback Adoree' Jackson came to the rescue with a sideline interception at the USC 25 in which he did the splits in mid-air and managed to toe-tap with his right leg before falling out of bounds.

"Basically, I was thinking of (Michigan's) Charles Woodson, what he did against Michigan State -- caught it with one hand, brought it back down and got his feet in-bounds," Jackson said, referring to a play from 1997.

Montez made his third consecutive start in place of Sefo Liufau, who left the Michigan game Sept. 17 because of a right ankle injury. Montez completed 25 of 40 passes for 197 yards, with one touchdown and an interception. Liufau played briefly at the end of the first half when Montez had the wind knocked out of him.

USC had a 539-371 edge in total yards as it stayed in the Pac-12 South race by knocking off the division leader.

"Great defense," Helton said. "We tried to give that game away offensively."

NOTES: USC RT Zach Banner, who missed last week's game because of a sprained ankle, was not in uniform Saturday. He was replaced by Jordan Simmons. ... Trojans TE Taylor McNamara (back) did not dress. ... Colorado was seeking its second consecutive road win, something it hasn't accomplished since 2004. ... USC leading rusher Justin Davis left the game early in the third quarter with an ankle injury and did not return. ... Colorado has at least one takeaway in 19 consecutive games, the longest streak in the nation. ... The Buffaloes dropped to 0-11 all-time versus USC and 2-20-1 in games played in California. ... Colorado LB Kenneth Olugbode had 10 tackles, his fourth consecutive game reaching double digits. ... Former USC WR Marqise Lee led the Trojans out of the tunnel before the game.