Inter were held for the second consecutive Serie A match as a 1-1 draw at Lecce handed reigning champions Juventus an unexpected boost in the title race.
Antonio Conte's men started 2020 with an impressive defeat of Napoli, but they only took a point against Atalanta last week and again failed to register a victory on Sunday.
It had briefly appeared as though 20-year-old defender Alessandro Bastoni would prove the unlikely hero when he headed in the opener - his first goal for the club - three minutes after arriving from the bench.
However, Lecce captain Marco Mancosu steered past Samir Handanovic with 13 minutes remaining, meaning Juve can go four points clear at the summit by beating Parma later in the day.
Inter made a bright start but Mancosu was guilty of a glaring miss with seven minutes on the clock, blazing over after reaching Khouma Babacar's stooping header uncontested at the far post.
The busy Romelu Lukaku saw an opener ruled out for a foul and then teed up Marcelo Brozovic, whose curling effort wrongfooted Gabriel but bounced away off the right-hand post.
Lecce were frustrated in the final moments of the first half when a penalty decision in their favour was overturned, referee Piero Giacomelli visiting the pitchside monitor before deciding Stefano Sensi's arm was in a natural position at his side as he handled in the area.
An open, entertaining spell following the interval was punctuated only by routine stops at either end and Inter were slow to reclaim full control of the contest.
But Diego Godin was replaced by Bastoni 22 minutes from time and the new man directed a header from Cristiano Biraghi's left-wing cross inside the near post.
Lecce's response was strong, however, and Mancosu got in front of Bastoni to finish coolly from substitute Zan Majer's delivery.
Filippo Falco - another introduction from the bench - could have stolen all three points, but his 81st-minute free-kick hit the outside of the post.