Willian has conceded the strength of Manchester City means Chelsea will find it tough to retain the Premier League title this season.
The champions already site nine points behind Pep Guardiola's pace-setters after 11 games, having lost three times this season.
Chelsea stormed to the Premier League crown last term, winning a record-breaking 30 of 38 games in Antonio Conte's first campaign, but have faced sterner tests this season.
Willian acknowledges opposition teams have raised their game against Chelsea in light of their formidable form of last term and City pose the biggest threat to their crown.
"It's difficult," he told BBC's Football Focus when asked if Chelsea can retain the title.
"Manchester City are playing very well but I think we can fight for the title until the end."
He added: "It is difficult to play when you were champion last season and to fight for the title again as every team wants to win against Chelsea."
