Real Madrid defeated Chelsea in the day's other encounter in the Champions League.
Victor Osimhen was missing in action Wednesday night at San Siro as AC Milan laboured to a 1-0 victory over 10-man Napoli in the first leg of their Champions League quarter-final tie.
Ismael Bennacer scored the only goal of the match for Milan from a well-executed counter-attack.
Aside from suffering defeat, Napoli also had Andre-Frank Zambo Anguissa harshly sent off.
Just 10 days ago, Milan surprised many with their 4-0 thumping of Napoli in Naples, a game from which Osimhen was also conspicuously absent.
But despite the absence of their Nigerian talisman this time, Napoli made an aggressive start. In the opening 90 seconds, Rade Krunic gifted the ball to Kvicha Kvaratskhelia and Davide Calabria made the goal-line clearance.
There was great joy as Milan through a counter-attack broke the deadlock. Brahim Diaz turned between two defenders to sprint forward and tried to combine with Leao. But the over-hit return pass came to Ismael Bennacer to fire in at the back post.
While Milan desperately sought to increase their lead, they were met with stiff resistance from Napoli.
The 10-man Napoli still threatened with Maignan denying Di Lorenzo at the near top corner and moments later Mathias Olivera nodded a Di Lorenzo cross over from six yards.
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With Osimhen expected back in time for the second leg, Napoli will be sure to keep their mission back on track with a bigger margin victory.
Fall for Chelsea
In Wednesday's other tie, Real Madrid hammered 10-man Chelsea 2-0 in the first leg of their quarter-final tie.
Chelsea started brightly and could have gotten an early lead through Joao Felix, but Real Madrid grew into the game and took the lead halfway through the first half.
Dani Carvajal's brilliant through ball was met by Vinicius Jr, whose effort was saved, but Karim Benzema slotted into the empty net.
Real Madrid continued pushing for more in the second half.
Rodrygo was sent through on goal, before being pulled back by Ben Chilwell.
The Chelsea defender was given the marching orders, which gave the hosts a man advantage for the final 30 minutes.
They took advantage soon after, with Marco Asensio scoring a second from the edge of the box.
Though Ancelotti pushed for another goal to completely seize the advantage ahead of the second leg in London, Chelsea made sure the tie ended 2-0; meaning the tie is not completely won or lost.
