Shenandoah (11-6, 8-2 ODAC) rebounded from a 7-1 deficit to the Quakers (10-6, 8-2) to earn the OT win.
Freshman Erin Higgins scored what proved to the game-winner at the 1:09 mark of the first three-minute overtime session and then sophomore Miranda Rudacille added an insurance goal with 1:50 to play to give SU the victory.
The win secured the No. 2 seed in next week's tournament and coach Lindsey Lutz' club will now await the winner of the No. 3 vs No. 6 game (most likely Guilford vs Lynchburg) in the tournament semi-finals next Saturday.
Shenandoah will host that game on the Sprint Field at Shentel Stadium turf.
Trailing 7-1 after E'leyna Garcia scored for the Quakers at the 15:16 mark, the Hornets reeled off five of the final six goals of the opening stanza to cut the deficit to just two, 8-6, at the break.
After Rudacille scored to make it 8-7 just 3:01 into the second half, the hosts seemingly took control with a 5-0 run that gave Guilford a 13-7 advantage by the 16:09 mark.
Lutz' club then kicked it into another gear, ripping off seven consecutive goals to lead 14-13 with 1:40 remaining.
GC tied it just 16 seconds before the end of regulation, setting up the two teams to play the six-minute overtime session.
Guilford outshot SU 35-32 with Hornets goalie Ashley Cross making 10 saves.
Shenandoah had a 23-18 advantage in groundballs with the Quakers winning 22 of the game's 34 draws.
Official tournament pairings will be announced later Saturday.