Ismael Bennacer’s first-half goal gave AC Milan a slender advantage over Napoli in the first leg of their all-Italian Champions League quarter-final.
The Algeria midfielder struck home from the edge of the area after Braham Diaz broke superbly on the halfway line.
Napoli had Andre-Frank Zambo Anguissa sent off in the second half and were well held by an organised Milan.
Milan also came close to a second goal when Simon Kjaer’s header bounced off the underside of the bar.
Napoli, in the Champions League last eight for the first time, were straight out of the traps, demonstrating a belief that comes from being 16 points clear at the top of Serie A.
Georgia winger Khvicha Kvaratskhelia had their best chance of the game in the opening minute after a poor clearance fell at his feet, but his effort from eight yards out was blocked on the line.
Napoli, the highest scorers in the competition this season, continued to push, with former Fulham midfielder Anguissa and Piotr Zielinski having efforts tipped over the bar.
Milan, who won 4-0 at Napoli 10 days ago in Serie A, weathered the storm and then grew into the game as Portugal winger Rafael Leao broke forward from his own half and slid an effort inches wide of the far post.
The 40th-minute winner was sparked by former Manchester City youth product Diaz, who showed great skill to let the ball run and turned to take two Napoli midfielders out of the game.
Bennacer, who played once in two years for Arsenal, then met Leao’s cut-back with a left-footed finish, which was helped into the roof of the net by the outstretched foot of visiting goalkeeper Alex Meret.
Milan’s Danish defender Kjaer was extremely unlucky with his powerful first-half stoppage-time header bouncing about a yard from the line after it struck the bar.
The second half was scrappy, with chances at a premium, but Anguissa earned two yellow cards in less than four minutes.
The first was for pulling down Theo Hernandez as the Frenchman looked to hit Napoli on the counter, before Anguissa caught the Milan left-back again with a high boot in the 74th minute.
Napoli will also be without defender Kim Min-jae for the second leg, which takes place next Tuesday, after he was booked for dissent in a frustrating second period.
Despite being a man down, though, Napoli almost found a late equaliser but Mike Maignan did superbly to claw Giovanni di Lorenzo’s effort behind for a corner.
“It is so important to have a positive result,” Milan forward Diaz, named man of the match, told reporters.
“Not conceding a goal was great. We need to play to win next week.”