Holders Donegal survived a stirring Monaghan fightback to advance to the Ulster Football semi-finals with a 0-23 to 0-21 win in Clones.
Having beaten Derry in the preliminary round, Donegal looked on course for another convincing win as they built a six-point lead at half-time.
But Monaghan roared back in the second half with three two-pointers in the space of six third-quarter minutes from Conor McCarthy, Jack McCarron and Micheal Bannigan.
Donegal managed to push themselves back into a four-point cushion in the closing stages, but a Ryan McAnespie score and Dessie Ward's two-pointer left the game in the balance.