DURHAM, N.C. -- Duke didn't play at the highest level, but the top-ranked Blue Devils were good enough anyway Friday night.

They also vowed to do better.

Wendell Carter Jr. scored 20 points and Duke had enough weapons to repel stubborn Southern in a 78-61 victory at Cameron Indoor Stadium.

"It was one of those days, everything didn't go as smooth as it usually does," Duke guard Trevon Duval said. "It still goes on the count (for a victory)."

Marvin Bagley III's 19 points and 11 rebounds -- before fouling out with 2:46 left -- also boosted the Blue Devils (4-0), who three nights earlier knocked off second-ranked Michigan State in Chicago.

Grayson Allen and Duval each added 10 points

"We weren't sharp," said Duke coach Mike Krzyzewski, whose team had 15 turnovers and 14 assists. "There wasn't a good flow. There was something missing."

Duke shot 44.6 percent for the field. The Blue Devils allowed Southern to grab 13 offensive rebounds, including eight in the first half.

"That was not our best game," Bagley said. "We played terrible. We've got to be better than that."

Duke began to take control by scoring 15 of the final 20 points of the first half for a 42-32 edge.

Jamar Sandifer scored 14 points and reserve Torrey Mayo had 11 for Southern (0-4), which trailed 65-53 with less than nine minutes left.

"We had a lot of guys step up to the plate," Southern interim coach Morris Scott said. "I thought we had a lot of guys who fought as a unit."

Only Bagley and Carter, a pair of freshman forwards, scored for the Blue Devils as they built a 19-13 lead across the game's first 10 minutes. They combined to shoot 14-for-21 from the field for the game.

The Jaguars were even at 27-27 with less than 51/2 minutes to play in the first half before Duke rattled off the next 10 points.

Shortly after, Bagley picked up his third foul and exited with 12 points.

Southern stayed within striking range despite team scoring leader Jared Sam going without a first-half point.

"We're still trying to find ourselves, but I think this can help us try to be a championship team," Scott said.

It was a ragged first half for Duke. Duval, a freshman point guard who had a total of four turnovers in the first three games, was charged with two in the opening 12 minutes.

"We didn't play very well," Krzyzewski said. "Part of it, a big part of it, was (Southern) played well."

This was the first Southern opponent to finish with less than 80 points. It was a season-low point total for the Blue Devils.

"We can do better on offense," said Allen, a senior who was 0-for-6 on 3-pointers after pouring in a career-high 37 points against Michigan State.

NOTES: Duke's homecourt winning streak against nonconference opponents increased to 135 games. ... Duke hadn't started four freshmen in a game since the 1982-83 season, but the Blue Devils have had a four-freshman, one-senior starting five in all four games this season. ... Duke associate head coach Jeff Capel was back with the team after missing Tuesday's game following the death of his father. A pregame moment of silence was observed. ... This was the second meeting between the teams, with Duke winning its 2006 NCAA Tournament opener against the Jaguars at the Greensboro Coliseum. ... Duke is back in action Monday night at home against Furman in an on-campus game that's part of the PK80 event. ... Southern remains in Durham for its fifth consecutive road game with Sunday's game at North Carolina Central.