Manchester City took revenge for their Champions League final loss to Chelsea and secured a big three points in the early running for the Premier League title with a 1-0 win at Stamford Bridge.
Gabriel Jesus' deflected effort eight minutes into the second-half separated the sides, but City should have been more convincing winners as they laid down a marker after losing to Chelsea three times towards the end of last season.
Pep Guardiola was fiercely criticised for his decision to start without a natural holding midfielder when the sides last met in May.
That error was rectified with the return of Rodri and the Spaniard's poise helped Guardiola's men completely dominate possesion.
Jesus got the break City's performance deserved to open the scoring on 53 minutes as his effort deflected off Jorginho to leave Mendy flat-footed. But there could have been plenty more but for a combination of wasteful finishing and some brilliant last-ditch defending.
Tuchel's response was to send on Kai Havertz, the hero of Porto with the only goal, but the German was well offside when he teed up Lukaku for a tap in.
But the Premier League champions' profligacy was not punished and United's shock 1-0 home defeat to Aston Villa capped a stunning Saturday for City.
In the day’s other match, Bruno Fernandes missed a stoppage-time penalty as Aston Villa beat Manchester United 1-0 at Old Trafford to prevent the Red Devils moving top of the Premier League.
The Portuguese midfielder blasted his 93rd-minute spot-kick over the bar minutes after Kortney Hause's late headed goal, which earned Villa their first win against United since December 2009.