Serie A leaders Juventus suffered a shock 2-1 defeat at Hellas Verona as the battle for the Scudetto took another twist.
Douglas Costa saw an early effort rattle the crossbar before the hosts - unbeaten in seven league games - had a goal ruled out by VAR for offside after Marash Kumbulla's glancing header from a free-kick.
Cristiano Ronaldo struck the post late in the first half and then broke the deadlock following a swift counter attack on 65 minutes.
Verona, though, were level with 15 minutes left when Fabio Borini latched onto an error from Miralem Pjanic and produced a cool, angled finish.
The home fans were screaming for a penalty after Leonardo Bonucci deflected a header from Marash Kumbulla onto the crossbar with a raised arm.
The referee reviewed the incident on the pitchside monitor, and awarded a spot-kick, which substitute Giampaolo Pazzini crashed past Wojciech Szczesny to cause an upset.
Inter could now go level on points with Juventus at the top of the table - if they win the Milan derby on Sunday, while Lazio can also close to within a point.
Earlier, Atalanta had consolidated their place in the top four after coming from behind to win 2-1 at Fiorentina, while Torino started life under new boss Moreno Longo with a 3-1 home defeat to Sampdoria and finished the match with 10 men.