Internazionale's Nicolo Barella scored one goal and set up another to help secure a 2-0 victory over champions Juventus and send his side joint-top of Serie A while putting a major dent in the Turin side's title hopes.
Former Juve midfielder Arturo Vidal scored his first league goal for Inter when he headed in an inviting Barella cross after 12 minutes.
Antonio Conte's side grabbed a deserved second after the break when Barella raced onto a long Alessandro Bastoni pass and finished clinically into the top corner, after which the victory was never in doubt as Juve failed to threaten a comeback.
Klinsmann: This was Arturo Vidal's game for Inter Milan
From before the match to the final whistle, Jurgen Klinsmann says Arturo Vidal dominated the day for Inter
It was Conte's first Serie A win on his fourth attempt against the club he previously represented as a player and coach, and the result leaves Inter level on 40 points with league leaders AC Milan, who visit Cagliari on Monday.
Juventus have a game in hand over Inter but are fifth with 33 points, leaving Andrea Pirlo's side with a lot of ground to make up in their quest for a 10th consecutive Serie A title.
"Juve have won the Scudetto for many years, but we've shown we're at the level to fight for the title," Vidal told Sky Sport.
"This success gives us the confidence to continue working hard and thinking about winning the league. We think we can go all the way."
The Italy international's teasing cross left Vidal with a simple header from close range to open the scoring.
Juventus approached the match as the team in form after winning three consecutive league games, but Pirlo's side struggled to break down the home defence and Ronaldo was reduced to speculative efforts from distance.
Barella got his goal when he hared after a 60-metre pass by Bastoni to finish in the 52nd minute, while Martinez was close to making it three when his curled effort flew wide.
Pirlo criticised his players' attitude and said they could not have performed worse in the loss.
"Our attitude was wrong from the start," Pirlo told Sky Italia. "If you don't have the anger to win the duels in these matches, it becomes difficult. We never stepped foot on the pitch.
"It was an ugly match because we weren't expecting it. We couldn't have done worse than this."
"Our ambitions remain the same," he added. "It's a slip-up against a strong opponent. These things can happen over a long season, but you can't have this attitude at a team like Juventus.
"We had to come here with a certain ambition of playing our own game and instead we were too fearful."
Conte, who Pirlo played under at Juventus as they won three league titles together between 2011 and 2014, was pleased with the win.
"To win against a team like Juventus, who are an important reference point in Italy, you need to play a perfect match," Conte said.
"The players did well to put what we prepared out on the pitch and I'm very happy for them as these games raise your self-confidence and let you know you're on the right track.
"They knew that Juventus have dominated for nine years. The gap between them and the others still exists, but we're working to reduce it from every point of view."