BYU took an early 11-4 lead but found itself plagued by foul trouble. Brandon Davies, Charles Abouo, Matt Carlino and Nate Austin picked up two fouls apiece in the first half. Davies played just eight minutes over the first 20 minutes.
A 6-0 run by the Toreros tied the game at 24-24, but the Cougars responded with a 7-0 run of their own, which included a 3-pointer by Anson Winder.
But San Diego outscored BYU 10-3 over the final four minutes of the half and tied the game again, 34-34.
Hartsock, wearing a brace on his injured left knee, entered the game at the 15:02 mark to rousing cheers of BYU fans. He scored four points in the first half.
San Diego outscored the Cougars in the paint, 20-10, in the first half, and outrebounded BYU, 16-13.
Now, the Cougars take on Gonzaga, which defeated BYU in Spokane, Wash., last week.
"It's really exciting for the fans," Rose said. "When we joined the league, these are the games everybody anticipated."
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