Zac Frischmon's seventh goal of the season 3:04 into overtime sent Mercyhurst onto the final of the Atlantic Hockey Tournament and eliminated top seed Robert Morris.
RMU, the defending champion, outshot the Lakers 62-29, but freshman goaltender Brandon Wildung made 59 saves to keep the game close. Chris Bodo tied the score for Mercyhurst with 34.4 seconds left in the game and the goaltender pulled for an extra attacker.
Mercyhurst took a 2-0 lead in the first period on goals from Kyle Just and Ryan Misiak, but Robert Morris charged back with three consecutive goals. Zac Lynch scored his 18th of the season to cut the deficit in half before the first intermission.
Hobey Baker Award finalist Cody Wydo scored his 20th of the season in the second period to even the score heading to the third.
Scott Jacklin gave Robert Morris its first lead of the game with 5:19 left in the third period. It gave the top seed the advantage, but the Colonials were unable to hold on, setting the stage for Mercyhurst's heroics.
Mercyhurst will face the winner of Friday's second semifinal between RIT and Canisius. The championship game will be at 7:05 ET Saturday at Blue Cross Center in Rochester, NY.