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Both sides were fairly even in terms of possession and attacking endeavour in the early stages, though Stefan Kiessling came close for Leverkusen with a stinging 20-yard drive just after the quarter-hour mark.
Bellarabi was next to threaten for the visitors 23 minutes in when he marauded in from the left and beat his markers before taking an extra touch and unleashing a shot that flew just high of the target.
There was a burst of action just before the half-hour mark when Lars Bender unleashed a thunderous effort from the edge of the box which cannoned against the left post, before Miroslav Klose struck the woodwork a minute later.
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Both sides went in level at the break as few chances came by either team's way, with the next concrete effort not coming until the hour mark when Hakan Calhanoglu's free kick was acrobatically saved by Etrit Berisha.
Admir Mehmedi was next to threaten in the 64th minute when he let rip with an effort from the edge of the box, but the ball flew just a few inches wide of the right post.
From then on both teams stepped up a gear in terms of attacking endeavour as Kiessling had a goal ruled out for offside and Keita forced a smart save from Bernd Leno midway through the second half.
It wouldn't be long before the deadlock was broken, and it went the way of Lazio, as their young Spanish striker struck the winner courtesy of a hazing solo run finished off with a strike off the post and into the net.