Bayer Leverkusen eased into the Europa League semi-finals after crushing hosts Union Saint-Gilloise of Belgium 4-1 on Thursday for a 5-2 aggregate win that stretched their unbeaten run to 12 straight matches across all competitions.

The result means Leverkusen, the last German team left in any European competition this season, have reached their first European semi-final in 21 years while ending Union's fairytale maiden run in the Europa League.

"The mentality of the team was the key tonight," said Leverkusen coach Xabi Alonso, who has turned around their season since taking over in October.

"Obviously the early first goal by Moussa Diaby was very helpful. But overall we did it very well.

"Now the team is ready to fight in a semi-final. We are a step closer to reaching the final but I'll only talk about the semi-final," said the Spaniard.

Leverkusen, who face AS Roma in the last four, proved extremely efficient, needing just 66 seconds for Diaby to round keeper Anthony Moris and finish a quick move by the Germans to open the scoring.

They added another goal on the break in the 38th with Mitchel Bakker volleying in at the far post before they were gifted a third on the hour courtesy of a big Moris blunder.

Under pressure from Bakker, the keeper sent the ball straight into the path of Jeremie Frimpong who only had to kick it into the empty goal.

The Belgians did breath some life into their game, cutting the deficit with Casper Terho's fine shot in the 65th, but could not find another goal despite more pressure

Leverkusen made sure they snapped their opponents' nine-game unbeaten run when Adam Hlozek pounced on a weak Moris save to make it 4-1 in the 79th.