The Portuguese star Ronaldo scored 451 goals in 438 games with Real Madrid, including a record 120 in Champions League play.
The 33-year-old then doubled the hosts' advantage on 65 minutes, this time producing a superb finish after a swift counter-attack.
The altercation was deemed to be worthy of a yellow card by the referee, but Costa was later sent off after VAR caught an elbow, headbutt and spit from the Brazilian global.
The Portugese striker scored two goals in a 2-1 win over Sassuolo to put the Italian giants back at the top of the Serie A.
I can't confirm that Costa spit into Di Francesco's mouth, but it sure looks like he did.
"Sassuolo fought hard and as they became exhausted later on, we took full control of the game". "This is football, but winning is most important".
Cristiano Ronaldo's double - his first goals in Serie A - proved the difference at Allianz Stadium, but it was Costa's very bad conduct in the closing stages that will be remembered. I really wanted to score, so I'm happy.
Ronaldo was delighted to get off the mark for his new club.
Costa now faces an extended ban with coach Massimiliano Allegri annoyed with the forward for "rising to provocation".
"You just had to see the first three games and how close Ronaldo went to scoring", he said.
Cristiano Ronaldo finally broke his duck for Juventus on Sunday, bagging both goals in a 2-1 win over Sassuolo. While the opener was arguably his easiest goal, the second goal was the goal to talk about.
"I don't expect Ronaldo to do more or less than what he did in the first three games, everyone was waiting for Ronaldo and he set up (Mario) Mandzukic".