The Brazil international set the Blues back £30m when he arrived in west London from Anzhi Makhachkala four years ago.
But he's repaid that hefty transfer fee by scoring 31 goals in 188 appearances and by being one of their most consistent performers.
Willian helped Chelsea win the Premier League last season - the second title and third trophy of his stint in England.
And the 29-year-old has now opened up on how it feels to be playing for Antonio Conte's side.
Willian told FourFourTwo: "I've won titles and made history at Chelsea.
"I've experienced incredible moments at one of the biggest clubs in the world.
"I think I managed to keep improving my football even during difficult periods for the team. I've managed to maintain a high level of performance.
"I’m very happy at Chelsea. I just try to improve year on year. And it’s been happening."
Willian was linked with a move to join his old Chelsea boss Jose Mourinho at Manchester United this summer.
However, the chances of an Old Trafford reunion with the Portuguese appear slim.
Willian will be hoping to help Chelsea get back to winning ways when they travel to Crystal Palace on Saturday October 14.
Conte will be without Alvaro Morata for the Selhurst Park visit due to a hamstring injury suffered in Saturday's defeat to Manchester City.
