AC Milan missed the chance to pile pressure on local rivals Inter in the Serie A title fight after falling to a 1-0 defeat at Lazio on Sunday, as Como beat 10-man Roma 2-1 to increase their hopes of a shock Champions League qualification.
Faced with the chance of cutting Inter's league lead to five points, Milan fluffed their lines and fell to a second defeat in four matches.
Gustav Isaksen's calm finish midway through the second half allowed Inter to increase the gap separating them from Milan to eight points even though they only managed to draw 1-1 with Atalanta on Saturday.
It was a huge let-off for Inter, who lost the Milan derby last weekend but with nine matches left in the season remain big favourites to reclaim the league title from Napoli.
"Everyone was talking about the Scudetto after Inter drew but you have to be realistic," said Milan coach Massimiliano Allegri.
"We're doing good things this season and we have to concentrate on our objective which is (qualifying for) the Champions League otherwise we risk ruining what we've built."
Milan are still on course for that goal as they are seven points ahead of fifth-placed Juventus.
But they were slow out of the traps at a loud Stadio Olimpico where Lazio's hardcore supporters returned after weeks of staying away in protest against the Rome club's direction under long-time owner Claudio Lotito.
Netherlands attacker Kenneth Taylor smashed a shot off the crossbar as Lazio dominated the opening half, before Milan took control after the break without properly testing rookie goalkeeper Edoardo Motta.
Maurizio Sarri was sent off in a tense finale as the Lazio coach exploded at officials for the length of stoppage time, with his team desperately trying to hold on for the points.
- COMO'S CHAMPIONS LEAGUE CHARGE -
Trailing at the break to Donyell Malen's early penalty, Como showed why they have shaken things up in Italy's top flight by coming back to beat a key rival for the final spot in Europe's elite club competition.
Cesc Fabregas's tiny but wealthy team are fourth, a point above Juve thanks to Tasos Douvikas' low drive in the 59th minute and Diego Carlos – who gave away Roma's penalty – poking home the winner in front of delighted home fans 20 minutes later.
"I'm pleased with the desire and mentality shown by the team," said Fabregas.
"We're a young team and we still have a lot to learn... we can't let ourselves get big-headed because of this win."
Between Como's two goals Roma defender Wesley was sent off for two bookable offences as the away side failed to deal with a lakeside onslaught in the second half.
Roma coach Gian Piero Gasperini was livid with referee Davide Massa's decision to show Wesley a second yellow card as the Brazilian barely touched Assane Diao who nonetheless fell to the turf as he charged towards goal.
"It was nothing, you can see from images that he does nothing," Gasperini told Sky.
"It's not the first time Como find themselves in that situation, they're looking for them, looking a little bit too hard for them in my opinion.
"But that's what football has become."
Como have never played European football in their turbulent history, which until they were taken over by tobacco giants Djarum in 2019 featured more bankruptcies than on-field success.
Roma, in sixth, haven't qualified for the Champions League since 2018 and trail Como by three points with nine matches left remaining in the season.