Lautaro Martinez's first-half brace put Inter Milan top of Serie A on Sunday with a 2-1 win over SPAL after champions Juventus were held 2-2 at home by Sassuolo.
"We didn't use our heads," lamented Juventus boss Maurizio Sarri after his side lost their first points at home this season.
Cristiano Ronaldo scored from the spot to rescue a point but Juventus slipped one point behind Antonio Conte's Inter Milan.
Argentinian Martinez put Inter ahead after 16 minutes at the San Siro, and then connected with Antonio Candreva's cross before the break to head in his eighth league goal this season.
Mattia Valoti pulled a goal back, but second-from-bottom SPAL fell to their ninth defeat.
Conte has guided Chinese-owned Inter to 12 victories in 14 league games, and they reclaimed the top spot which they had held until their 2-1 defeat to Juventus on October 6.
In Turin, Leonardo Bonucci struck early for the eight-time reigning Serie A champions but Jeremie Boga and Francesco Caputo hit back for Sassuolo before Ronaldo's point-saving intervention.
Sassuolo, in 12th, claimed their first ever point at Juventus, and held Sarri's unbeaten champions to their third draw of the season.
"We got into trouble on our own, we didn't use our heads in the first half, as the goals conceded show," said Sarri.
"Every time we lost the ball we left ourselves open. We had a first half without application and little energy from the character point of view.
"But we were coming off two games in which we used up a lot of energy against Atalanta and Atletico Madrid.
"We woke up after going behind."
Ronaldo started up front, having missed last weekend's league game with a slight knee problem, as Paulo Dybala, who scored in their Champions League win over Atletico Madrid, started on the bench.
Eighteen-year-old Stefano Turati made an impressive debut in the Sassuolo goal, and pulled off a string of fine saves, denying Ronaldo, Gonzalo Higuain and Dybala late.
But Juventus counterpart Gianluigi Buffon had a nightmare. The 41-year-old, making just his fifth start this season, was at fault in Caputo's goal just after the break.
Bonucci got Juventus off the mark with a long-range effort on 20 minutes.
Three minutes later Boga pounced on a Caputo cross and sent it past an out-rushing Buffon from an angle.
- Ronaldo lacking brilliance -
Caputo added a second following a mix-up between Juan Cuadrado and Matthijs De Ligt in the Juventus defence, with Buffon then fumbling the effort into the back of the net.
Turati got down low to deny Higuain after getting his hand to send a Ronaldo free kick over the bar.
Ronaldo's penalty pulled the hosts level on 68 minutes after Filippo Romagna fouled substitute Dybala.
It was the Portuguese star's fifth Serie A goal this season.
"Ronaldo has regained stamina, the next step will be to recover brilliance, but the road is right," insisted Sarri.
Sassuolo coach Roberto De Zerbi hailed "an important point" for his team.
"You need courage when playing here, otherwise you're dead before kick-off," said De Zerbi.
He also heaped praise on Turati for his solid performance.
"I had no doubts about him, he is the classic goalkeeper a little crazy. Before the game, I told him to enjoy the moment -- to be 18 playing against these champions is wonderful.
"From the way he looked at me at the end of the game, I think he enjoyed it."
Lazio consolidated third place -- seven points behind Inter -- with a 3-0 win over Udinese thanks to a double from Serie A top scorer Ciro Immobile, including one from the spot, to bring his tally to 17 goals this season, and a Luis Alberto penalty.
AC Milan, struggling in 11th, got their first win in a month 1-0 at Parma with Theo Hernandez scoring the winning goal two minutes from time.
AS Roma, in fifth, travel to Verona later Sunday, with Napoli, in seventh, hosting Bologna.
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