Radja Nainggolan returned to the Stadio Meazza to haunt former club Inter as his late goal grabbed Cagliari a 1-1 draw with the Nerazzurri.
Ashley Young, the first Englishman to play in Serie A for Inter since Paul Ince, made an almost immediate impact, taking less than 30 minutes to get his first assist, as he set up Lautaro Martinez for the opener.
But, it was Nainggolan who broke Inter hearts with just 12 minutes to play as his deflected effort rolled into the net. Then tempers flared in the final moments with Lautaro given his marching orders in stoppage time.
As a result, Inter do move to three points behind Juventus at the top of Serie A, but the Bianconeri face Napoli later on Sunday. Cagliari meanwhile stay in sixth place.
Cagliari started well as Nahitan Nandez found Giovanni Simeone in the penalty area, but the Argentine smashed over from close range.
Nicolo Barella did have the ball in the net thanks to a cheeky lobbed header which looped over Alessio Cragno and in off the crossbar, but it was ruled out for offside.
The opener did come as Young swung in a brilliant ball from the right which landed right on the head of Lautaro who made no mistake, nodding past Cragno.
There did look to be a small push on Sebastian Walukiewicz from the Argentine No.10, but after consulting VAR, referee Gianluca Manganiello confirmed that the goal would stand.
Lautaro, who looked dangerous throughout, dribbled into the box and fired a low shot to the near post which was saved by the feet of Cragno.
Samir Handanovic was then called into action himself, just before half time, to produce a brilliant save from Paolo Farago.
After the break, Inter had two good chances. First Romelu Lukaku fired into the side-netting, then a low drive from Lautaro was stopped by Cragno.
Stefano Sensi managed to take the ball through the Cagliari backline, but his eventual shot rolled just wide of the post.
A cross-come-shot from Luca Pellegrini had to be punched over the bar by Handanovic.
Nainggolan levelled the scoreline with less than 15 minutes left to play, with his strike from 25 yards out taking a wicked deflection off Alessandro Bastoni and rolling in off the post.
Lukaku took the ball from inside his own half but put his shot just wide with Cragno rooted to the spot.
From there Inter went searching for a winner, but never really troubled Cragno in the final moments and things went from bad to worse as Lautaro was sent off in time added on for continuing his protestations at receiving a yellow card.