Since then it is City who have dominated the competition, taking the crown in five of the past six campaigns.
Chelsea did edge out City in the 2021 Champions League final but there has been plenty of turbulence at the London club since, and they currently sit 10th in the table.
Guardiola, who takes his side to Stamford Bridge on Sunday, said: “Chelsea remain one of the more important teams in England.
“It’s true that in the Premier League they were not close to winning it in these last seasons but they’re still one of the strongest teams, there’s no doubts about that.
“I see that in the last games they are alive, they have good spirit, they are aggressive, they have a good pattern. The quality is there.
“It’s one of the toughest games that we have during the season. Sooner or later Chelsea will be there fighting for the titles, and I guess sooner than later.”
Chelsea have had a rocky start to the campaign under new manager Mauricio Pochettino but Guardiola has no doubt they will come on strong.
He said: “It’s normal. They are getting better. Every manager needs his time – it’s a new club, new players but they’ll be a top contender because the quality in all departments is there.”
Two of Chelsea’s better performers this season have been the former City pair of Cole Palmer and Raheem Sterling.
City academy product Palmer, 21, switched clubs in August having grown frustrated at a lack of opportunities at the Etihad Stadium.
Guardiola said: “The reason why was that he wanted game time. Now he has it, so congratulations.
“He’s a guy who came from the academy and if he is having success it’s good for him and we wish him the best. I’m really pleased for him.”
Sterling won four Premier League titles and the FA Cup at City and Guardiola recognises him as a dangerman this weekend.
Guardiola said: “He’s playing really good, he’s back in his best moments, best performance, (from) what I’ve seen lately.
“He always creates incredible dangers for the opponents.”
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