Francesco Caputo scored a quick-fire brace to pave the way for Sassuolo's first-ever Italian top-flight victory over Roma as the capital club slumped to a 4-2 loss on Saturday.
Paulo Fonseca's Roma remain fourth but their Champions League berth is now at risk with Atalanta just a point behind in fifth before they play struggling Genoa on Sunday.
"The players lost their heads," said Fonseca.
"The truth is we didn't really play in the first half. And it's impossible to win if we defend like this.
"It's difficult to explain such a difference between the game against Lazio (1-1 last weekend), I don't know why we played like this today." Caputo scored twice within the first quarter of an hour in Emilia-Romagna, as Roberto De Zerbi's side pulled Roma apart.
Filip Djuricic slid in the third on 26 minutes. Skipper Edin Dzeko scored his 100th Roma goal on 55 minutes, but his team were hit by Lorenzo Pellegrini's sending off for a second yellow card 14 minutes later.
A Jeremie Boga handball threw Roma a lifeline with Jordan Veretout making no mistake from the spot on 73 minutes.
But Ivorian Boga immediately made amends, cutting inside to rifle in a superb fourth goal.
Sassuolo, sitting 13th, had never beaten Roma in 13 top-flight meetings and had lost the reverse fixture 4-2 in September at the Stadio Olimpico.
It was Roma's fourth defeat this year.
"Today we were a complete team, everything went well," said De Zerbi.
"If we always play in this way this team has no limits."
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