The Portuguese raced through to net within the opening seven seconds to break Paolo Poggi’s record, set in 2001, and maintain Milan’s unbeaten start to the season.
Alexis Saelemaekers doubled the advantage midway through the first half, and despite Domenico Berardi’s late freekick, Milan sealed victory to remain top of the table.
Milan came flying out of the blocks and were incredibly ahead in less than seven seconds, as they broke the deadlock from kick-off. Hakan Calhanoglu drove forward from the centre circle and slipped Leao through to slot home and give the Rossoneri a dream start.
The visitors had the ball in the back of the net again before the 10 minute mark as Leao returned the favour to Calhanoglu, racing into the box with a clever stepover before teeing up the Turkish midfielder to side-foot in. However, a lengthy VAR check ruled Leao offside in the build up and the goal was chalked off.
Sassuolo offered a threat on the counter as Milan pushed forward, with Gregoire Defrel testing Gianluigi Donnarumma from the edge of the box after a quick break and Berardi getting in behind.
Yet it was Milan who proved devastating on the break, as Theo Hernandez charged up the wing after a Sassuolo corner was cleared and cut into the box to tee up Saelemaekers, doubling the Rossoneri’s lead.
Whilst Sassuolo came close through Hamed Traore’s bending effort from the edge of the box, Milan ought to have wrapped up victory on the stroke of halftime, only for Leao to slice wide from close range after Calhanoglu’s reverse pass.
In the second half Sassuolo began brightly, particularly with the introduction of Francesco Caputo at the interval, and the substitute threaded Traore in but his tame shot was easily stopped.
Similarly, Berardi found space on the edge of the box but sent his effort whistling over the crossbar, as the Neroverdi chased a way back into the match.
The hosts remained on the front foot as Milan lost their early spark and the Rossoneri’s attempts to counter proved fruitless, with Hernandez and Jens Petter Hauge overrunning the ball on the break.
However, the best opportunity of the second half fell to Saelemaekers after Leao showed good strength in the box to pull the ball back. The Belgian blazed well over the crossbar as the ball flicked up to him, must to his frustration.
At the other end, Mehdi Bourabia’s daisy cutter stung Donnarumma’s fingertips, before Milan raced forward and Hauge forced an excellent save from Andrea Consigli.
With less than a minute remaining, Alessio Romagnoli clipped Jeremie Boga on the edge of the box and Berardi stepped up to fire in the freekick and give Sassuolo a lifeline. The Neroverdi captain’s effort deflected off Hauge’s head to wrongfoot Donnarumma at the near post.
Ultimately, Milan hung on to continue their unbeaten start to the domestic season at the top of the table, whilst Sassuolo remain sixth.