Kevin De Bruyne felt he was never given a fair chance during an 18-month spell at Chelsea where he played nine games before asking to be sold for the sake of his career.
Nearly four years later, the Belgium midfielder returned to Stamford Bridge as a highly coveted and more rounded player with Manchester City, and showed his former club exactly what it lost.
De Bruyne helped City assert control of the biggest game of the English Premier League season so far, then secured a 1-0 win with a 25m shot that flew into the top corner in the 67th minute.
"What happened, happened," De Bruyne said. "I have no regrets coming here - it made me stronger in many ways."
Playing in a deeper role as a centre midfielder rather than as the winger who arrived at Chelsea in August 2012, De Bruyne is proving a crucial part of a City team that has won six of its seven league games - drawing the other one - and is unbeaten in 10 matches in all competitions.
Even without Sergio Aguero and Benjamin Mendy, who sustained injuries in the past week, City dominated Chelsea in what left back Fabien Delph described as a "big message" to the rest of the league.
De Bruyne's goal came when he collected a pass from defence with a first-time flick to Gabriel Jesus. The striker laid the ball off instantly to De Bruyne, who surged through to the edge of the box before unleashing a left-footed shot that flew past goalkeeper Thibaut Courtois.
"He made absolutely everything," City manager Pep Guardiola said. "He scores, assists, fights like a humble guy. I'm so happy for him, as I know how happy he is."
It was Chelsea's first loss in the league since its opening-day home defeat to Burnley. Chelsea is six points behind the Manchester clubs.
City stayed in first place on goal difference from Manchester United, who won 4-0 against a beleaguered Crystal Palace side that is still without a point or goal.
Romelu Lukaku scored his 11th goal in 10 games for United this season. They have now had four 4-0 wins in the league.
Juan Mata opened the scoring inside three minutes and Marouane Fellaini netted either side of halftime.
Another player who cannot stop scoring is Harry Kane, who finished September with 13 goals from eight games by scoring twice in Tottenham's 4-0 win at Huddersfield.
Kane extended his bewildering run of goals in September by scoring twice in the first 23 minutes at Huddersfield, either side of Ben Davies' goal.
Moussa Sissoko added a fourth goal in injury time for Tottenham, who are a point ahead of Chelsea in third place.
There were also wins for West Ham and Stoke, courtesy of late goals, while Richarlison's effort in the fifth minute of injury time snared a point for Watford at West Bromwich Albion. Leicester held on to draw at dominant Bournemouth 0-0.
• Hibernian gave Celtic a scare in their Scottish Premier League 2-2 draw. Celtic's 57-match domestic unbeaten run looked in danger when John McGinn's second-half double put the visitors 2-1 up in the 77th minute, but Callum McGregor grabbed his second of the game three minutes later to deny Hibs a notable win.
Adam Rooney's hat-trick helped Aberdeen moved level on points at the top with a 3-0 win at home to St Johnstone.