The Nerazzurri tend to bring out the best in the Neroverdi, as they came into the game with a record of seven wins in 13 matches since their promotion to Serie A in 2013.
Francesco Caputo gave the visitors an early lead, but a Romelu Lukaku penalty and Cristiano Biraghi strike put Antonio Conte’s side in charge at half time.
The hosts couldn’t hold on during a sensational second half that saw Roberto Gagliardini miss a sitter, Domenico Berardi convert a penalty, Borja Valero put Inter back in front late on and substitute Magnani net a last-minute equaliser.
The result could be costly for Inter’s Scudetto hopes, as it leaves them eight points adrift of Serie A leaders Juventus with 11 rounds remaining.
Both coaches heavily rotated their starting line-ups, Inter making five changes, including a start for Alexis Sanchez in place of Lautaro Martinez, and Sassuolo replacing all seven players who made up their defence and midfield in the 4-1 defeat to Atalanta.
The hosts were cut open with ease after just four minutes, when Filip Djuricic drove forward and slid a pass into the path of Caputo, who slid a neat finish under Samir Handanovic for his 14th Serie A goal of the season.
Inter couldn’t muster a reaction as they struggled to break through the visiting defence and often left themselves open on the counter, but they finally burst into life in the final five minutes of the first half.
Milan Skriniar cunningly nipped in front of Jeremie Boga and went down after being caught on the ankle, allowing Lukaku to fire home the equaliser from the resulting penalty.
With just seconds remaining before the half-time whistle, Biraghi played a neat one-two with Sanchez before blasting a shot into the roof of the net to complete Inter’s comeback.
The Milanese side had a golden opportunity to wrap up the points just after the hour mark when Andrea Consigli parried Lukaku’s shot into the path of Gagliardini, but the unmarked midfielder somehow managed to blast a finish off the bar from five yards with an empty goal in front of him.
The visitors soon took advantage thanks to a disastrous introduction to the game from substitute Ashley Young. The wing-back was fortunate not to concede a penalty after Mert Muldur’s flick struck his arm, but just moments later he tripped the Turk to allow Berardi to blast home the equaliser from the spot.
The frantic pace of the game didn’t slow down as it entered the final five minutes, as Lukaku had a rebound strike disallowed for offside before Valero tapped in at the back post from a delicious Antonio Candreva freekick to nudge Inter back in front with four minutes remaining.
Sassuolo refused to give up and found a sensational leveller in the final minute of regulation time, as substitute Magnani tapped in a cross from point-blank range.
With just 30 seconds left on the clock, things ended on a sour note for Inter as Milan Skriniar was sent off for a second yellow card.