Santa Clara (23-14) will face Iona or East Tennessee State, who play Saturday. SMU (20-15), finished its first 20-win season since 1999-2000.
Mustangs coach Kevin Doherty is happy not to deal with Foster again.
"I've been doing this for 20 years, and I can't remember anyone being that good," Doherty said. "He's an unbelievable shooter. I don't know how we could've guarded him any better."
With the game tied at 51 with 7:44 left, Foster took over. He scored 13 points on a 21-4 run to close out the game.
Foster sealed the victory with a four-point play with 2:14 left. With two seconds left on the shot clock, he drained a 3 from the left wing while getting fouled to give the Broncos a commanding 66-54 lead.
"I actually get fouled a lot on 3s," said Foster, who was one-point shy of his career high. "I've gotten used to it, and just have to focus on making the shot. I had a feeling I was going to get fouled there."
Foster finished 13 of 23 from the field, including 8 of 10 on two-point shots.
"I still want him to be better," Santa Clara coach Kerry Keating said. "He's figuring it out, and he took better two-point shots. He's a special player, and special players can carry a team in March."
"I thought the play was excessive," Doherty said. "It's hard when your best player only plays 11 minutes."