Downey came in averaging more than 32 points per game and hadn't scored fewer than 29 in three Southeastern Conference games, but he was ineffective against the Rebels' backcourt. He did score 25 points, but seven came in the final 2 minutes with the game out of reach.
South Carolina native Murphy Holloway added 14 points and 12 rebounds for the Rebels (14-4, 2-2 SEC), who held the Gamecocks (11-7, 2-2) to 32.1 percent shooting from the field and outscored them 36-20 in the paint.
Terrico White had 11 points for Ole Miss and Eniel Polynice had 10, including four as the Rebels put together a late game-clinching 11-3 run.
Downey was 9 of 10 from the free throw line and had five assists. But he also had four turnovers and missed 13 of 20 field goals.
The Rebels built an early lead after a slow start. The Gamecocks opened with a basket and held Ole Miss scoreless for the first 2:11. But once Polynice stole the ball at halfcourt and dropped in a layup to jumpstart the Rebels' offense, the team rolled to a 16-4 run fueled by six early South Carolina turnovers and 3-pointers from Warren and Trevor Gaskins.
Downey ended the spurt with a runner and started to make plays. But each time Downey did something, Warren had an answer during a stretch when the Rebels opened the lead to 28-15 with 7:54 left in the half.
After Downey's runner, Warren took the inbounds pass and weaved his way through the South Carolina defense for a layup. Downey found Johndre Johnson with a nice pass for a dunk and Warren answered with his own to Greg Buckner.
White and Warren hit consecutive 3s a few minutes later to push Ole Miss' lead to 36-20, the biggest of the game, and the Rebels led 38-23 at halftime.
The Gamecocks opened the second half with the game plan of driving the lane and it worked immediately. They started with a 13-5 run that included five free throws and two 3-pointers from Ramon Galloway, who finished with 11 points.
South Carolina ended up with a significant free throw advantage over Ole Miss (the Gamecocks were 16 of 22, while the Rebels were 7 of 11) and the team whittled the lead down to 50-44 and 53-47 with 5½ minutes left by taking advantage of turnovers. But the Gamecocks would get no closer as the Rebels stepped up the defensive pressure.