Carlsen has always had a tenacity that allows him to come back from positions other players would give up on thinking to conserve effort to fight another day. Mental strength and stamina to stay in the fight has always been something that made him who he is.

You also can't underestimate physical stamina. Kasparov in his 5-3 result against Karpov in 1984-85 was eventually halted due to Karpov's exhaustion and losing 18kg over the match period.