Andre Villas-Boas has said that fatigue cost his Zenit St Petersburg side in last night's 2-1 Europa League defeat to Sevilla.

Aleksandr Ryazantsev gave the hosts a potentially crucial away goal on 29 minutes but goals from Carlos Bacca and Denis Suarez flipped the game on its head from the 73rd minute onwards.

"The first half was very good for us," Villas-Boas told the club's official website. "We played well, possessed the ball, were confident on defence. Of course, the fact that we were able to score also influenced the situation.

"There were chances to increase the lead, however we capitalised only on a single one of our chances. From the 60th minute, we started to have some problems. We looked worn out. We were forced to make some subs and this influenced the outcome of the match.

"Sevilla increased their control and had a lot of corners. It's pretty clear that our opponents gave out all their strength and that will make a difference in the return leg. The stadium wasn't the easiest to play in, and when our opponent scored their first goal, its emotions influenced both them and us as well."

Zenit could win 1-0 in next week's return leg in Russia and progress into the semi-finals.