Annie Bourne posted 15 points and 13 rebounds, securing her spot in the SU record books for the most career rebounds.
An 8-0 run early in the game gave the Pirates a cushion to work from, but Denison evened the score once again, 18-18, with eight minutes to play.
Denison then pulled ahead by eight, 31-23, with 1:48 remaining in the half and sent the Pirates into the locker room down six, 34-28.
Denison scored the first basket of the second half, but Southwestern got hot to tie the game 40-40 on Olivia Podaras' jumper seven minutes into the frame.
The Pirates' run continued until they led by eight again, 48-40 with 11 minutes remaining. Southwestern never lost the lead as they held off Denison down the stretch.
Jordan Owens led the Pirates with 18 points on 7-14 shooting from the floor, 1-3 from three point range and 3-3 from the free throw line. Madison Eschle also had 15 points to match Bourne's offensive output and Podaras added in 11. Jordan Holmes posted 16 points to lead Denison.
The Pirates, now 1-0 on the season, head to the championship game where they will face the winner of Washington University and Rose Hulman on Saturday for a 7pm tipoff.