Other clubs from various leagues who are participating in UEL also have their own domestic competitions that they are also involved in and not only EPL clubs. Any club that can stay strong for so long in a good forn will perform better than the rest. Fatigue is a norm for players due to the unlimited matches that they play during the season.
Fatigue cannot be considered a norm because if it does, then there will be no need to bother about it enough to want to rest player so as to prevent it from happening too often. Most times some teams will even put in measures and not let their players who are key to them to embark on such so they do not get to suffer injury back at their national team and coming back to their club to take it.
Why I said fatigue is a norm is because if a player features in all the matches, there is a high probability that he will be affected by fatigue. However, this is not true for all players. Some are use to play more matches than the other. To overcome the problem of fatigue, players need to be rotated. This is why it's important to have enough players on the bench for easy rotation.