Anzhi Makhachkala sold Willian to Chelsea for a fee thought to be £25.5m at the end of last month.
On the move to Stamford Bridge, Willian said: "Things just happened. Chelsea came with an offer and everyone knows that I wanted to play for the club since I was at Shakhtar.
"It shook me but I didn't think twice. As both presidents (Suleyman Kerimov and Roman Abramovich) are friends, that helped too. It was a very easy negotiation.
"The Brazilians called me, especially David Luiz, who told me to come quickly. I was very happy to have them around and they will certainly help me a lot.
"When I had a day off, I came to watch the club's games, so I am more than happy for having made this transfer become true."
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