Duvan Zapata, Lucas Torreira and Gian Marco Ferrari put Sampdoria 3-0 up in the second half before Juventus threatened an astonishing stoppage-time comeback with a penalty from Gonzalo Higuain and a strike from Paulo Dybala.
Sampdoria, meanwhile, moved five points behind Juventus with a game in hand on most other teams.
The champions were without defender Andrea Barzagli and goalkeeper Gianluigi Buffon, who were recovering from the disappointment of Italy’s failure to qualify for the World Cup, and Sampdoria made the most of their absences.
Benevento again came agonisingly close to claiming their first Serie A point, before a late goal saw them lose 2-1 at home to Sassuolo.
Samuel Armenteros netted in the 65th for Benevento to cancel out Alessandro Matri’s opener but the hosts’ hopes of getting something from the match were dashed when defender Gaetano Letizia was sent off two minutes later following a second booking.
Benevento clung on and it appeared as if their luck was about to change when Domenico Berardi hit the crossbar with a stoppage-time penalty, but Federico Peluso netted the winner two minutes later. Benevento have no points from their first 13 matches in the top flight and have twice before conceded a stoppage-time winner.
Talented youngster Federico Chiesa celebrated signing a new deal at Fiorentina with a late goal to help La Viola draw 1-1 at SPAL.
Chievo also drew 1-1 at Torino, where home striker Andrea Belotti had his penalty saved.