Tuscon, Ariz. ? The curse of No. 2 claimed another victim.

Second-ranked Oregon lost Heisman Trophy candidate Dennis Dixon to a knee injury and never recovered in a 34-24 upset by Arizona on Thursday night.

Oregon (8-2, 5-2 Pac-10) became the fifth No. 2 team to lose since Oct. 6, following USC, California, South Florida and Boston College. The Ducks’ defeat could open the door for third-ranked Oklahoma in the Associated Press Top 25, and for No. 3 Kansas in the Bowl Championship Series standings.

Dixon, Oregon’s gifted starting quarterback, hurt his left knee in the first quarter, about seven minutes after he ran 39 yards for a touchdown. Dixon crumpled to the turf without being touched and took Oregon’s national championship hopes with him.

“My foot got planted in the ground, and my knee went the other way,” Dixon told ESPN toward the end of the game.

Antoine Cason returned a punt 56 yards for a touchdown and an interception 42 yards for another score as the Wildcats shook up the national title race on a cool night in the desert. Red-clad students poured out of the grandstand as the Wildcats (5-6, 4-4) ambushed a ranked team in Arizona Stadium for the fourth straight season under coach Mike Stoops.

Brady Leaf, who replaced Dixon, completed 22 of 46 passes for 163 yards and threw two interceptions. Dixon finished 5-of-8 for 62 yards and threw an interception. He carried twice for 34 yards.

Arizona quarterback Willie Tuitama completed 21 of 39 passes for 266 yards and two touchdowns and was intercepted once. Mike Thomas caught two touchdown passes for the Wildcats.

Oregon’s Jonathan Stewart carried 28 times for 131 yards.

Early on, Oregon appeared to be on cruise control with Dixon running its potent offense, which came in averaging 42.8 points per game, fifth in the nation.

The Ducks opened the game with a brisk seven-play, 70-yard scoring drive. On fourth-and-three at the Arizona 39, Dixon froze the defense with a fake to Stewart, burst through a hole in the right side and ran untouched to the end zone. Ed Dickson ran for the two-point conversion to put the Ducks ahead 8-0.