Partizan Belgrade, the 1992 winners of Europe's premier club basketball competition, edged closer to the Top 16 with a shock 79-77 away success at Fenerbahce Istanbul, who had sealed their passage previously.
With 11 teams through to the next phase, five more spots will be sealed in next week's final round of the initial stage.
Guard Alessandro Gentile nailed two free throws with 17 seconds remaining for triple Euroleague winners Milan, who will host the May 16-18 Final Four.
He finished with 16 points, one less than team mate and the game's top scorer Gani Lawal, who nailed six of eight shots from the field and all five from the free-throw line.
"The team was ready mentally and tactically for a game like this and we did a great job finding the energy and resources in both offence and defence," Milan coach Luca Banchi told the competition's official website (www.euroleague.net).
"We are happy to advance to the Top 16 and it's a great prize for our hard work," he said.
Fenerbahce were strong favourites against Partizan but the Serbian title holders stunned them with outstanding long-range shooting, hitting 11 of 18 three-pointers to give themselves a fighting chance of progressing from a tough preliminary pool.
Partizan, who have a 3-6 record, will go qualify if they beat six-time winners CSKA at home in their final game, with the Russians already through alongside Fenerbahce and double winners Barcelona.
"I think we played our best game of the season and I hope we can emulate this performance against CSKA," said Partizan guard Bogdan Bogdanovic, who sank five three-pointers and totalled 19 points.
Forward Tarence Kinsey netted 20 and captain Dragan Milosavljevic chipped in with 18 as Partizan squandered a 17-point lead before rallying in the final quarter of a rip-roaring rollercoaster.
Elsewhere, Spaniards Unicaja Malaga beat Bayern Munich 77-72 at home and Zalgiris Kaunas edged out Anadolu Efes Istanbul 65-63 as both sides also moved closer to the Top 16.
The second group stage will feature two groups of eight teams, with the top four from each pool advancing to the best-of-five quarter-final playoff series. The winners will qualify for the Final Four.