The Blues play at defending champions Leicester holding a five point lead at the top of the table.
However, they lost their last match to Tottenham – a result which saw an end to their 13 game winning streak –
Chelsea also have tough games against title rivals Liverpool and Arsenal coming up.
Chelsea remain firm favourites to lift the trophy in May, but Willian insists they can’t afford to become complacent.
He said: “In the Premier League you never know what can happen in the future – in one month, in two games even. Everything can change and that’s why we have to stay at the same level we have been.
“Teams are trying even harder to beat us, which is why now especially, we are in a moment where we have to prepare ourselves for that.
“The sides coming against us want to stop us and maybe they will play like West Brom did, with eight players behind the ball, staying back and wanting to play on the counter attack.
“We have to stay focussed because it’s difficult to beat those teams sometimes. In those kind of games when you have one, two or three chances, you have to score when those chances come.”