Inter Milan's faint hopes of catching runaway Serie A leaders Napoli suffered a huge blow when they fell to a shock 2-1 defeat at relegation-threatened Spezia on Friday.
Spezia struck through Daniel Maldini and M'Bala Nzola's penalty to beat Inter for the first time, with Romelu Lukaku's equaliser from the spot counting for nothing in the end.
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"In football, you get no discounts and we paid for those errors. It's not right that Inter lose to Spezia," Inter manager Simone Inzaghi told DAZN. "We won't sleep tonight.
"I take responsibility for the defeat, but the lads gave their all tonight. We just weren't clinical enough.
"This is football, defeats make you stronger. We had a few too many this season, especially away from home, and naturally the result is unsatisfactory."
Napoli lead Inter by 15 points before the leaders host Atalanta on Saturday. Simone Inzaghi's side stay second with 50 points, only two points ahead of Lazio who visit Bologna.
AS Roma and AC Milan are three points off Inter before hosting Sassuolo and Salernitana respectively.
Inter had the chance to go in front early in the match when Danilo D'Ambrosio was fouled by defender Mattia Caldara and a penalty was awarded to the visitors after a VAR check.
However, Lautaro Martinez's attempt in the 14th minute was superbly saved by Polish goalkeeper Bartlomiej Dragowski.
"I wouldn't focus on the penalty, but more the 20-30 shots on goal that we had tonight," Inzaghi added. "We should have done better with those. Then once we did equalise, we clearly shouldn't have conceded another penalty."
Spezia then took the lead themselves 10 minutes into the second half with their first shot on target when Nzola fought off two defenders inside the box before finding the unmarked Maldini, who fired into the bottom corner of the net.
Inter got another penalty after 83 minutes when Denzel Dumfries was fouled and this time Lukaku made no mistake.
But Nzola ruined Inter's night completely when he converted the game's third penalty three minutes from time after Dumfries ran into Spezia midfielder Viktor Kovalenko inside the box.
"Last season we did very differently away from home. I don't believe in bad luck in football, you earn your luck and we weren't determined enough in our finishing," Inzaghi said.
The win helped Spezia's bid to avoid the drop as they remain in 17th place but are now six points above the bottom three.