Fabian Ruiz's fine goal gave Napoli the edge in the first leg of their Coppa Italia semi-final against Inter Milan.
Ruiz curled a superb away goal from the edge of the area to send Antonio Conte's Serie A leaders to only their second domestic defeat of the season.
Former Chelsea boss Conte was booked during the second half as Inter failed to fashion any clear chances, and they must now win in Naples next month.
AC Milan face Juventus on Thursday at the San Siro in the second semi-final.
Inter came from 2-0 down to beat rivals AC at the weekend and go top of the table but they could not find a late rally on this occasion.
Ashley Young, Alexis Sanchez and Christian Eriksen were all absent from the starting line-up and Dries Mertens and Piotr Zielinski both went close for Napoli in the first half.
Away fans from the Naples region were banned from this match - a consequence of an incident in December 2018, when an Inter fan died of his injuries after being knocked down by a van amid violence before a match between the sides.
Despite that ban, there were still plenty of Milan-based Napoli supporters inside to celebrate the winner from Ruiz.
The midfielder played an elegant one-two on the edge of the box, and then bent a left-footed strike around Daniele Padelli and into the top corner.
Eriksen and Sanchez were introduced as Inter tried to respond, with on-loan Manchester United forward Sanchez failing to get a shot away when well placed and Romelu Lukaku, another former Premier League striker, seeing a scrambled effort well saved.
Eriksen then had a shot deflected wide but Napoli held on to take the lead into the second leg, which takes place on 5 March.
- 37SkriniarBooked at 3minsSubstituted forD'Ambrosioat 45'minutes
- 6de Vrij
- 11MosesSubstituted forSánchezat 74'minutes
- 12SensiSubstituted forEriksenat 66'minutes
- 25OspinaBooked at 85mins
- 22Di Lorenzo
- 44ManolasBooked at 51mins
- 6Mário RuiBooked at 77mins
- 20ZielinskiSubstituted forAllanat 82'minutes
- 7CallejónSubstituted forPolitanoat 78'minutes
- 14MertensSubstituted forMilikat 73'minutes
- Gianpaolo Calvarese
- Shots on Target