West Ham boss David Moyes warned they must not be complacent in the second-leg after their 1-1 draw at Europa Conference League opponents KAA Gent.
Danny Ings had West Ham ahead in the quarterfinal first-leg before Hugo Cuypers' equaliser.
Ahead of the return-leg in London, Moyes warned: "Gent are a threat. I think people sometimes think teams at this [UEFA Conference League] level are not good but they're all good teams.
"There are very few poor teams when you get to the quarter-finals and you're down to the last eight.
"People have worked really hard all season to get to this point and they don't want to give it up. I think Gent showed that.
"I remember coming away from Seville and we were disappointed to lose 1-0 [in the UEFA Europa League round of 16 first leg last season], but we knew we had a chance.
"We'd get the crowd behind us and we won it in extra-time. We've got a big game next Thursday now to try and reach the semi-finals, so let's hope we can do that."