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Rayo will take on Oliver Glasner's side on 27 May in Leipzig, but it needed Augusto Batalla to save a late penalty from Julio Enciso to ensure there was no nervy finish.

The visitor started brightly and only a stunning save from Mike Penders denied Alemao's header from Andrei Ratiu's teasing delivery into the box from finding the back of the net. 

Strasbourg struggled for any kind of rhythm in the first half and saw a poor giveaway lead to Unai Lopez's shot from distance, which forced Penders to parry the ball to safety. 

But Rayo's dominance was rewarded three minutes before the break by Alemao, who was the quickest to react to Florian Lejeune's saved volley before finishing first time from inside the six-yard box.  

Guela Doue came close to equalising straight from the resulting kick-off after latching onto a ball over the top, but his tame shot was repelled by Batalla to conclude a frustrating half for Gary O'Neil's men.  

Jorge de Frutos then fired a glorious opportunity wide early in the second half before Samir El Mourabet fired over, while Valentin Barco saw his shot brilliantly blocked by Pep Chavarría's sliding challenge at the other end. 

But Strasbourg threatened a grandstand ending to the game after Oscar Valentin handled Doue's goal-bound effort, but Enciso saw his spot-kick saved by Batalla before the Rayo goalkeeper Ismael Doukoure on the rebound to send the visitor through.