Serie A leaders Inter Milan fought back from a goal down to beat city rivals AC Milan 2-1 in a thrilling derby on Sunday.
Ronaldo, who left Inter to join Real Madrid in 2002, gave AC Milan a first-half lead with a left-footed drive.
The game turned in Inter's favour early in the second half when Julio Cruz came on as a substitute for Hernan Crespo. The Argentine forward netted the Nerazzurri's equaliser eleven seconds after coming on, and then set up the winner for Zlatan Ibrahimovic 15 minutes from time.
Inter's victory went some way to making up for their UEFA Champions League exit on Tuesday, and extended their lead over second-placed Roma to a massive 19 points. Inter have 73 points, followed by Roma on 54. Roma can reduce the deficit back to 16 points if they win at home to Udinese in Sunday's late match.
Ronaldo fires Milan in front
AC Milan went close to taking the lead in the 22nd minute at the San Siro, when Andrea Pirlo's dipping shot was tipped over the bar by Julio Cesar.
Crespo's curling shot bent inches past the post before his flicked header rolled agonisingly wide. The momentum stayed with Inter with Luis Figo driving fractionally wide.
Totally against the run of play, AC Milan went ahead five minutes before half time. Ronaldo, who had barely touched the ball, picked up the ball from Gennaro Gattuso before burying a low shot into the bottom corner from 25 yards.
Nelson Dida came to AC Milan's rescue five minutes after the restart, blocking Ibrahimovic's stinging shot with his legs.
Cruz inspires comeback win
Gattuso almost gave AC Milan a two-goal cushion with an angled shot before Crespo made way for Julio Cruz. The Argentina international immediately made his mark, slamming home the loose ball after Dida could only manage to get his fingertips to Ibrahimovic's low, right-wing cross.
Cruz then repaid the compliment, slicing his way through AC Milan's defence before cutting the ball back for Ibrahimovic, whose emphatic finish gave Dida no chance.
Inter coach Roberto Mancini refused to say that the title race was over, despite his team's commanding lead. "I don't want to talk about the Scudetto, but if we keep going like this it will be an extraordinary season and, I think, unrepeatable," he said.
Ronaldo admitted Inter were worthy winners. "We were better in the first half, but Inter deserved the victory," he said.
Asked if his goal meant more to him as it came against his former club and also silenced the hecklers, he replied. "It was a derby, but for me it was just like another match. It's always a joy to score, and the fact that it happened today didn't make it any more special."
Defeat for AC Milan ended their 15-match unbeaten league run and dented their hopes of a UEFA Champions League place for next season.
Lazio move alongside Palermo
Lazio claimed their fifth consecutive win with a 3-2 victory at Reggina, which saw them join Palermo in third place on 46 points. Palermo were held to a 1-1 draw at home to Fiorentina.
Empoli's push for a top-four finish was dealt a blow after they threw away a two-goal lead to draw 2-2 at home to Livorno.
The relegation scrap between Chievo and Messina ended in a 1-1 draw, as did the match between relegation-threatened Parma and Atalanta.
Ascoli claimed only their third win of the season with a 1-0 victory at Siena, while Torino pulled further away from the drop zone by defeating Catania 1-0.