Unai Emery says Thursday's Europa Conference League clash with Olympiacos is Aston Villa's "biggest challenge" of the season.

Villa are 4-2 down after a shock defeat in the first leg of the semi-final tie at Villa Park last week.

While they are pushing for a top-four finish in the Premier League, Emery believes attempting to overturn the deficit in Greece will prove to be Villa's biggest test yet.

"This is the challenge tomorrow, the biggest challenge we’re going to face this year, playing in this semi-final," Emery said.

"I will find the best performances tomorrow and really be competitive.

"Of course, it's going to be very difficult here away because the atmosphere supporting the home team could be more difficult.

"In 90 minutes, we have to be focused with everything tactically, being in control of our emotions, set-pieces as well, everything."

Olympiacos or Villa will face Fiorentina in the final on May 29, after the Serie A club overcame Club Brugge on Wednesday.