AC Milan conceded an injury-time goal as Atalanta twice fought back to claim a point in a 2-2 draw at the San Siro.
Gonzalo Higuain gave them an early lead only for Alejandro Gomez to level after the break, and when it seemed Giacomo Bonaventura had secured a win, the unmarked Emiliano Rigoni tapped in from close range.
Milan's perfect start had seen them take the lead in the second minute with the first real attack of the game, Gonzalo Higuain controlling a Suso cross before steering the ball low into the net.
They thought they had added a second when former Atalanta player Bonaventura headed home, only for his effort to be ruled out for offside following a video assistant referee (VAR) review.
Milan were having the better of the first period but enjoyed an escape just after the half-hour mark when Mario Pasalic failed to get a clean connection when he was left with a free header.
Bonaventura hit the post for Milan in the closing stages of the half, and they were made to regret their failure to add a second goal when, with 54 minutes gone, Gomez sent a neat finish past keeper Gianluigi Donnarumma.
But they responded less than 10 minutes later, Bonaventura making it third time lucky when he steered the ball beyond Pierluigi Gollini.
Robin Gosens and Marten De Roon had opportunities to snatch a point for Atalanta, the latter forcing a save from Donnarumma, but Rigoni was their hero in the second minute of added time.