Nicholas (7-3) led 10-7 at halftime but broke the game open on two Tyler Sagraves touchdown passes and a Mayes 5-yard TD run in the second half.
Sagraves, who completed 7 of 16 passes for 67 yards, hooked up with Joe Amendola on a 9-yard TD pass and Drew Moore on a 5-yarder.
Ritchie (7-4) opened the scoring on Devon Jones' 9-yard run in the first quarter.
The Grizzlies advance to quarterfinal play against No. 1 Wayne.
David Gray and Noah Spitzer scored two touchdowns each - Gray on runs of 14 and 7 yards and Spitzer on runs of 22 and 4 - as St. Marys (10-1) totaled 259 yards on the ground.
St. Marys will face today's Madonna-Tug Valley winner. Wirt, which rushed for minus-27 yards but passed for 129, ends the season at 7-4.
Magnolia's Stephen Rogalski returned the game's opening kickoff 99 yards for a touchdown, helping the Blue Eagles (10-1) to a 27-0 halftime lead.
Dakota Watson rushed nine times for 108 yards for Williamstown (6-5).
Magnolia will face the winner of today's Wheeling Central-East Hardy game in the quarterfinals.