Suso's missed penalty ultimately proved costly as AC Milan's Europa League hopes took a hit in a 2-1 defeat to lowly Empoli at San Siro.
Milan were aiming to build on last weekend's dramatic 2-2 draw against city rivals Inter and a win would have given them a five-point buffer over their rivals in Serie A.
However, despite dominating large periods of the first half - in which they were restricted to mainly long-range efforts - Levan Mchedlidze headed home in the 41st minute to put Empoli in front.
The home side had the chance to draw level when Empoli goalkeeper Lukasz Skorupski felled Mario Pasalic, but the goalkeeper made amends by keeping out Suso's effort.
Substitute Carlos Bacca then missed a great chance from close range and Empoli made Milan pay when Mame Thiam rifled in a second in the 67th minute.
Gianluca Lapadula, who scored twice in a 4-1 win over Empoli in this season's reverse fixture, halved the deficit with a well-placed effort but Milan were unable to repeat the heroics they produced against the Nerazzurri and stay sixth, just two points above rivals Inter and five adrift of Atalanta in fifth.
For Empoli, the win follows a fine victory over Fiorentina last week, but Crotone's triumph at Sampdoria means they stay just five points above their relegation rivals, who are 18th.
Milan were bright from the off and Gerard Deulofeu twice laid on Suso with presentable efforts on the edge of the area that the Spaniard failed to capitalise on.
Davide Calabria's low right-wing cross then picked out Lapadula, but the striker - in for Bacca as one of three Milan three changes - scuffed his shot at Skorupski from 12 yards.
The hosts almost shot themselves in the foot when Cristian Zapata undercooked his backpass, but Omar El Kaddouri failed to poke his shot past Gianluigi Donnarumma.
Milan continued to dominate possession, but were restricted to mainly long-range shots and they were undone before the break when El Kaddouri's corner from the left was powerfully headed home by Mchedlidze at the near post inside the six-yard box.
The second half continued in a similar fashion with Suso and Lapadula trying their luck from range.
Milan had a great chance to draw level on the hour when Skorupski made a hash of a Lapadula effort and clumsily felled Pasalic.
However, the goalkeeper kept out Suso's penalty with his foot low to his left and made a fine stop from Lapadula's follow up effort.
Montella threw on Bacca in a bid to find the equaliser and a scrappy corner fell kindly to him three yards from goal, but he could not sort out his feet and fluffed his shot.
And Empoli were 2-0 ahead in the 67th minute.
Veteran Massimo Maccarone brilliantly cushioned a header into Thiam's path and the forward stayed onside to emphatically hammer home on the half volley.
Milan quickly hit back when Lapadula picked up the ball on the right of the area, shuffled onto his left foot and placed his shot low into the far corner.
In a frantic finish, Donnarumma made a brilliant one-on-one stop to deny Maccarone, while at the other end Lucas Ocampos - who had earlier seen claims for a penalty for handball against Andres Tello waved away - smashed the crossbar for Milan, who fell short in their bid to earn a point.
Key Opta stats:
- AC Milan with this defeat have interrupted a home streak of four wins in a row in Serie A. Their last defeat at San Siro was February 5 versus Sampdoria.
- Empoli have won two league games in a row for the first time since December 2015.
- Empoli have won a match against AC Milan for the first time since October 2007 (D3, L3).
- Skorupski has saved four penalties in the current Serie A, only Samir Handanovic in the 2010-11 season did better (six).
- Three of the Lapadula's six goals in Serie A have arrived against Empoli.