Barca had 70 per cent of the possession in the first half, but Quique Setien’s side were unable to muster a single shot on target in the opening 45 minutes.
Lionel Messi was forced to drop deep and without injured forwards Luis Suarez and Ousmane Dembele, the Catalans struggled to find spaces in and around the Napoli area.
Napoli’s forwards offered only an intermittent threat on the counter-attack, but Gennaro Gattuso’s side took the lead with their first real opportunity after half an hour.
Junior Firpo lost the ball to Piotr Zielinski and the Pole sent a low ball across to left side of the area for Dries Martens, who checked back and curled a shot past a static Marc-Andre ter Stegen from the edge of the box – although the Belgian’s effort took a slight deflection off Gerard Pique’s chest on its way in.
Barcelona improved after the break and equalised after 57 minutes, when Sergio Busquets played a pinpoint pass through to Nelson Semedo and the Portuguese in turn crossed low for Antoine Griezmann to net the equaliser from close range.
Messi almost went through with a couple of runs after that, but Barca had Marc-Andre ter Stegen to thank for a super save from Jose Callejon.
Barca went down to 10 men in the closing stages after Arturo Vidal was sent off for two yellow cards in quick succession, following a crunching tackle and a angry reaction in which he clashed heads with Mario Rui.
The Catalans will also be without Sergio Busquets for the second leg after he was booked and saw Gerard Pique limp off in the closing stages, leaving Setien’s side down to the bare bones for the return in three weeks’ time.
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