City's first defeat on home soil from open play since the eve of the 2022 World Cup leaves them still five points adrift of Premier League leaders Liverpool, who play their game in hand at bottom-of-the-table Southampton on Sunday.
Guardiola has committed himself to turning around the poorest run of his managerial career after extending his contract until 2027 this week.
But hopes his new deal could revitalise a City side, that have won an unprecedented four consecutive Premier League titles, proved short-lived.
"In eight years we have never lived this kind of situation," said Guardiola, who joined the club in 2016.
"Now we have to live it and break it winning the next games, especially the next one. Now we see things in one way, maybe in a few weeks we see it differently."
Rodri paraded his Ballon d'Or trophy to the home fans before kick-off as his name was lit up across the Etihad pitch by firelight in an ostentatious show of support for the Spanish midfielder.
The 28-year-old is not expected to play again this season after suffering knee ligament damage in September and his absence has been at the heart of City's demise.
Guardiola's men did start strongly as Guglielmo Vicario denied Erling Haaland and the Norwegian had another shot deflected wide.
But the momentum of the match completely swung on Tottenham's first serious attack after 13 minutes.
Dejan Kulusevski's teasing cross picked out Maddison who applied a cool first-time finish.
"In this moment we are fragile defensively," added Guardiola.
"We started really well as normal but we could not score and then after that we conceded. After that we conceded some more which is difficult for our emotions right now."
Spurs inconsistency
Maddison had been frozen out of Postecoglou's starting line-up in recent weeks and was keen to prove a point to his manager.
Son Heung-min's inventive pass freed the England international inside the box and he dinked the ball over the advancing Ederson to double Spurs' lead.
Inconsistency has been the scourge of Tottenham's season so far.
A 2-1 home defeat to Ipswich before the international break saw Postecoglou's men slip to 10th in the table.
But they have now twice won comprehensively in Manchester after a 3-0 win over United at Old Trafford.
"It is significant because obviously they've got an unbelievable record here at home," said Postecoglou.
"We've reached these standards before. It's about now not dropping as we have in certain games."
Spurs quashed any idea of a comeback seven minutes into the second period.
The excellent Kulusevski crossed for Dominic Solanke and he patiently laid the ball back for Porro to blast home from close range.
Haaland clipped the bar from an almost impossible angle as City laboured to find a response.
Instead, they were hit with one final sucker punch as Timo Werner eased past Kyle Walker and crossed for Johnson to tap home at the far post.
The one glimmer of hope for City may be the return of Kevin De Bruyne after a two-month injury absence as a late substitute.
However, Guardiola has to find solutions fast if his side's title defence is to be maintained into the new year.
A daunting trip to face Liverpool at Anfield is next up in the Premier League, where City have not won in front of a crowd since 2003.