Inter's Lautaro Martinez fired into the net on 16 minutes and nodded in on 41 at a rainy San Siro, with Mattia Valoti storming into the box to pull one back for SPAL five minutes after the interval.

Inter are on 37 points from 14 games, one point above Juve, who lost the top place they had occupied since early October.

"When a team like Juventus drops points, you need to make sure that you make the most of that," Inter coach Antonio Conte said.

"But we need to look at ourselves, we need to keep moving down this path of growth."

Rain also fell at Turin's Allianz Stadium, where the unconvincing champions went ahead on 20 minutes from a low drive by captain Leonardo Bonucci.

Sassuolo levelled three minutes later as Jeremie Boga raced past Juve's static defenders to beat veteran Gianluigi Buffon with a clinical lob.

Francesco Caputo then capitalised on some poor Juventus play to fire a drive that Buffon, filling in for Wojciech Szczesny, let slip through into the goal.

Substitute Paulo Dybala was floored for a penalty converted on 68 minutes by Cristiano Ronaldo.

The Portugal ace had previously seen a free kick saved by Turati, who made his debut at 18 and also denied late attempts from Dybala and Aaron Ramsey.

Turati's youth was highlighted by the appearance of Buffon at the other end.

The 41-year-old had made 160 Serie A appearances before Turati was born.

"I am speechless. I did not expect it, it was immensely emotional," said Turati about his first top-flight appearance as Sassuolo were forced to use their third-choice due to injuries.

"The save on Ronaldo (from his free-kick) was the most important. It was an honour to play against Buffon. He complimented me after the game."

Juve remained unbeaten in the league while Sassuolo are 14th.

"It was not a negative match even though we were not concentrated in the first half and we made mistakes," Juve coach Maurizio Sarri said.

"It was different in the second half; after falling behind we played strongly."

Lazio captain Ciro Immobile turned in a Sergej Milinkovic-Savic pass and converted a first-half penalty to strengthen his top scorer tally to 17 goals as the third-placed side beat Udinese 3-0.

Roma won 3-1 at Verona to move up to fourth.

Justin Kluivert, Diego Perotti (pen) and Henrikh Mkhitaryan got the goals for the Romans while Davide Faraoni replied for the hosts.

Seventh-placed Napoli's winless run extended to eight matches after they lost 2-1 at home to Bologna.

AC Milan snatched a late 1-0 win at Parma via a Theo Hernandez goal and are now 11th on 17 points, one behind the hosts.

Australian Associated Press