After a week marked by the highly anticipated arrivals of former Juventus forward Mario Mandžukić and Chelsea defender Fikayo Tomori, AC Milan was expected to celebrate winning Serie A's halfway title in style.
Instead, the Rossoneri were handed a humbling 3-0 defeat at home by Atalanta on Saturday and saw their lead over city rival Inter Milan trimmed to two points.
The only consolation was that Milan still secured the "Winter title" — which seven times out of 10 leads to the actual Italian league championship.
Cristian Romero, Josip Iličić and Duván Zapata scored for Atalanta -- the surprise Champions League quarterfinalist last season.
It was Milan's second defeat of the season but also the second in four matches, having been beaten 3-1 by nine-time defending champion Juventus earlier this month, also at home.
Inter drew 0-0 at Udinese under driving rain.
Third-place Roma, which trails Milan by six points, gained some relief after a rough spell with a 4-3 win over visiting Spezia.
At the San Siro, Atalanta took control midway through the first half with a diving header from Romero.
A bloodied Iličić then converted a penalty shortly after the break after getting elbowed in the face by Franck Kessié.
Mandžukić came on in the second half and nearly scored immediately, forcing a save from close range from Atalanta goalkeeper Pierluigi Gollini.
But Milan produced little more and Zapata then hit the post before he finished off a counterattack in the 77th to seal it.
In Udine, Inter coach Antonio Conte was sent off in added time for protesting.
Lorenzo Pellegrini scored the winner for Roma two minutes into stoppage time after former Giallorossi winger Daniele Verde had equalized for Spezia in the 90th.
Borja Mayoral scored twice and Rick Karsdorp also found the target for the Giallorossi, who were without captain Edin Džeko due to an apparent feud with coach Paulo Fonseca.
Roma was beaten 3-0 by Lazio in last week's league derby and then lost 4-2 to Spezia in the Italian Cup on Tuesday. On Friday, the Cup defeat result was changed to a 3-0 loss by the league judge due to an impermissible sixth substitution that Roma used.
The recent results have led to speculation that Fonseca is at risk of losing his job.
Roma players celebrated wildly after Pellegrini's goal, which was the product of work from Leonardo Spinazzola and Bruno Peres, whose chest pass left a clear look for Pellegrini from close range.
Pellegrini then ran over to hug Fonseca
"It was an emotional moment," Fonseca said. "That run and that embrace show that we're all together. It was a nice team moment."
Still, Fonseca wouldn't address the issue with Džeko, who watched the match from the tribune.
"I don't want to say anything further," Fonseca said. "What counts today is what was done today -- the guys obtained a great team victory."
Roberto Piccoli and Diego Farias also scored for Spezia, which is playing in the top division for the first time. Spezia was left four points above the drop zone.