The work of an encouraging first half was undone by the second, as the Marauders squandered a lead en route to a 4-2 loss in their final game of the regular season on Sunday.
McMaster had led 2-1 at the break, through goals from Gersi Xhuti and Stefan Scholz, but conceded three times in the second half as Guelph swept the season series.
Chris Jacovou scored twice in that frame, while Jace Kotsopoulos and Jalen Noronha also had goals for the Gryphons in the win.
With the loss, the Marauders finish the season with an 11-5 record, good for third in the OUA West standings. McMaster will now host the Western Mustangs in an divisional quarterfinal on Wednesday, October 28.
Xhuti opened the scoring on Sunday from the penalty spot, punishing the Gryphons after Marco Gennaccaro was brought down in the area in the 14th minute. Guelph hit back just nine minutes later, as Kotsopoulos got his name on the scoresheet to level the match.
The goals continued to flow, as Scholz restored McMaster's lead six minutes on, heading past the goalkeeper in the 29th.
That advantage held into the halftime break, but the Gryphons emerged from the locker room as the more organized side. Jacovou scored his first of the half in the 59th to level the game once again, before Noronha put the Gryphons ahead in the 67th.
With McMaster rarely threatening to create an equalizer, Jacovou struck for his second goal to put the match beyond reach just a minute before the onset of injury time.