Metz had only found the back of the net three times this season, but it only took the visitors 13 minutes to break the deadlock.
Clinically combining, Cheikh Sabaly’s defence-splitting pass put strike-partner Habib Diallo in on goal, and the forward who netted a brace when Metz were last victorious in this fixture in August 2019, comfortably beat the onrushing Philipp Kohn to give the visitors a surprise lead.
One could have easily become two for Metz just seven minutes later, as Koffi Kouao’s effort looked to be heading for the top corner, yet Kihn was alert to the impending danger as he pulled off a stunning save.
That stop proved pivotal as Monaco equalised with their very first shot on target just before the half-hour mark.
Expertly positioned, Mika Biereth left Metz goalkeeper Jonathan Fischer with no chance, as he converted Takumi Minamino’s brilliant ball into the box from close range to conclude the first half with the game level once more.
That scoreline didn’t remain unchanged for much longer, as Monaco ruthlessly turned the match on its head immediately from the restart.
Replacing Paris Brunner at the break, Fati made an immediate impact by expertly slotting home Lamine Camara’s pass from deep inside the area.
That lead was close to being extended further in the 56th minute, but Fischer produced a sublime reaction save to keep out Maghnes Akliouche’s effort.
Despite dominating the opening quarter-hour of the second half, Monaco were pegged back by Metz in the 67th minute from the penalty spot.
Minamino’s needless foul on Kouao allowed captain Gauthier Hein to lead by example, confidently firing home from 12 yards to give his side some momentum and the chance to cause an early-season upset.
With a point to prove after an unsuccessful loan spell at Brighton & Hove Albion, Fati headed home in the 83rd minute from Krepin Diatta’s delivery.
Metz’s disappointing trip was compounded not long after as Kouao inadvertently headed Camara’s corner past Fischer, sealing the points for Monaco.
Substitute George Ilenikhena added a fine fifth goal for Monaco in injury time to cap off a third successive win in Ligue 1, lifting Adi Hutter’s side up three places to second position in the table and softening the blow of a heavy midweek UEFA Champions League defeat to Club Brugge.
Meanwhile, Metz remain rooted to the foot of the standings, without a win (D1, L4) since being promoted back to the top flight this season.
Flashscore Man of the Match: Ansu Fati (AS Monaco)