Napoli's hard work nearly paid off.
After hitting a wall for the entire first half, Barcelona found a way through Napoli's bulwark to earn a valuable 1-1 draw at the San Paolo on Tuesday.
Barcelona will now return to the Camp Nou for the second leg of the last-16 tie with a narrow advantage on away goals. A win on March 18 will guarantee them passage to the quarterfinals.
Gennaro Gattuso's side had done an impeccable job keeping the visitors at bay before Griezmann finally struck. Barcelona failed to record a shot on target in the opening 45 minutes, and Griezmann only had six touches. They spent the majority of the time playing harmless passes back and forth and never seemed likely to score.
Napoli, meanwhile, maximized their small chunk of possession. Piotr Zielinski stole the ball off Junior Firpo in a dangerous area and played a cross to Mertens, who curled in his 121st goal for the club. The Belgian is now tied with Marek Hamsik as Napoli's all-time leading scorer.
The game opened up after the interval. Messi and Griezmann suddenly created space for themselves and flustered Napoli's back line. The Frenchman buried his chance and gave his team something to take from a cagey first leg.
Bad news arrived late in the second half when Arturo Vidal was sent off for a stray tackle on Mario Rui. Vidal was booked for the challenge and issued a second yellow card after locking horns with the Portuguese full-back.