Napoli clipped the woodwork twice and were largely ineffective, as Cagliari bided their time before a shock last-gasp Lucas Castro winner.
The Partenopei had won every competitive game this season other than the last-gasp 4-3 defeat to Juventus and Carlo Ancelotti was taking full advantage of investments with squad rotation. Kalidou Koulibaly was rested, but Dries Mertens, Allan, Alex Meret and Jose Callejon returned. Cagliari had back-to-back 3-1 victories over Parma and Genoa, but the injury crisis ruled out Radja Nainggolan, Leonardo Pavoletti, Alessio Cragno and Paolo Faragò. Luca Cigarini was also carrying a knock, so Christian Oliva started with ex-Napoli midfielder Marko Rog.
The Sardinians had lost each of their last six Serie A meetings with Napoli, home and away, going into the Stadio San Paolo this evening.
Giovanni Simeone had some promising early opportunities on the counter-attack, but Hirving Lozano’s strike was charged down after a good run from Dries Mertens.
Mario Rui ballooned over from point-blank range, but was grateful to see his blushes spared by the offside flag.
Mertens threaded through to release Lorenzo Insigne, but Robin Olsen rushed off his line to parry with his body one-on-one.
Artur Ionita and Kostas Manolas both threatened with headers after the restart, while Mertens clipped the upright as he met a Di Lorenzo cross at the near post.
Incredibly, moments later, Mertens also made contact with the other upright, this time a curler from the edge of the box.
Koulibaly came off the bench and wasted a free header, while Fernando Llorente also nodded just wide from a promising position.
Incredibly, Cagliari defended stoutly and took the lead against the run of play in the final minutes. Nahitan Nandez whipped in a cross from the right and substitute Lucas Castro was left completely unmarked for a free header from close range. Koulibaly was so furious at the goal that he was shown a straight red card for dissent.
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Sent off: Koulibaly 88 (N)