This is why you can't be reactionary when a team is outplaying another team early im the game by using a lot of intensity, aggression and pressing. A lot of teams can do it but no team can sustain it over 90 minutes and they'll gas out in the end.
Remember when people were celebrating the downfall of Madrid and La Liga and the dominance of the PL when Liverpool went up 2:0 at home only to lose 2-5 later on?