City failed to beat Chelsea in the league last season - drawing 4-4 at Stamford Bridge and then 1-1 at the Etihad Stadium.
But goals from Erling Haaland and former Chelsea star Mateo Kovacic saw Guardiola's side begin their quest for a fifth straight league title with an impressive 2-0 win in west London.
City's preparation for the new season was far from ideal. With many of their star men given extra time off after their international exertions, Guardiola took a youthful squad on tour of the USA, where they lost to Celtic and Milan, before drawing with Barcelona.
That tour ended with a 4-2 friendly victory over Chelsea in Ohio on August 3. But 15 days on, Guardiola claimed that beating the Blues for real had been beyond his expectations.
Pep Guardiola pictured (left) kissing Kevin De Bruyne (right) after Man City beat Chelsea 2-0
'I really didn't expect it,' Guardiola told BBC Match of the Day. 'I know what these guys can do, but the first day and with a lack of training...'
He later continued: 'Last season we could not win at Stamford Bridge so this is good news.
'The bad news is we still have 65, 70 games left. But it is good to start with a win for the confidence.
'We tried to prepare for the game, we had two or three days to prepare, but we solved this lack of many things with our commitment. Everyone was at their maximum and this was enough.
'You cannot imagine how privileged and fortunate I am to live these experiences with this incredible group of players. They are all friends, their families are all with them most of the time. It's unbelievable for me.'
Guardiola added: 'We have an advantage, it's our ninth season together so we know each other well. Of course there are mistakes you have to improve but we talked, the difference in being champions was our body language, how did we behave?
'It's the bad moments in the season and in the games that define the big teams. For me, it is an honour to be in charge of these guys. I'm so fortunate, I could not say more.
'There is a plan, defensive, offensive. But afterwards it's how we celebrate the good moments and come back from the bad moments.
'That's what defines these teams and no one talks about it. For a while we could never win at Stamford Bridge. This is a big step.'
City recorded three consecutive league victories at Stamford Bridge between January 2021 and January 2023.
But prior to that, Guardiola had seen his side lose three times in four visits to Chelsea.