The Brazilian was instrumental in Tuesday's 3-1 away win, producing a goal and two assists as Chelsea bounced back from Saturday's shock 1-0 loss at West Ham.
Although Chelsea's results have been inconsistent, while City have won 14 successive league games to establish a dominant cushion, Willian believes the Blues will continue to push hard to defend their crown.
"I think we had a great game, we had possession and tried to play the ball," Willian told BT Sport. "We created a lot of chances and we have to carry on.
"Always when I play I want to play well and make a difference. Of course I want to play every game, but only 11 can start.
"We know it’s difficult [to win the title], but we have to fight until the end. We are Chelsea, we are a big club — we have to fight."
Eden Hazard accepted it was necessary for Chelsea to show a reaction after going down 1-0 at the London Stadium last time out.
"We had a bad game against West Ham — when things like this happen we need to bounce back," Hazard told BT Sport.
"We did well today, the fans are happy, we are happy — let’s move forward for the game against Southampton. We’ll be ready and will fight to the end.
"When we play like this it is easy to play — good movement, the manager wanted us to move a lot and not always stay in the same position.
"We have a lot of games in a short period of time, but since I came to Chelsea it’s always been the same."