Guyn, who finished with 18 points, drove for a layup to tie the game at 57-57 with 28 seconds left in regulation. He made a jumper to start the extra period and ETSU (12-7, 5-1) never trailed in overtime.
Mercer held a 27-23 halftime advantage but Nigel Holley had a layup and T.J. Cromer scored four straight as ETSU took a 35-32 lead with 13:33 remaining. Mercer came back to take a 54-47 lead with 4:10 left. ETSU had an 11-3 run to close regulation and force overtime.
Cromer led the Buccaneers with 20 points and Isaac Banks grabbed 14 rebounds.
Phillip Leonard had 18 points and eight rebounds for Mercer (14-5, 4-2).