City have been in fine form recently, beating European champions Bayern Munich, Arsenal and Leicester City in the Champions League, Premier League and Capital One Cup respectively.
Manuel Pellegrini's side are fourth in the Premier League and remain on course for success in Europe and the Capital One Cup, but Kompany knows the busy December fixture schedule - City face Fulham, Liverpool and Crystal Palace over the next nine days - will have huge ramifications on the remainder of the season.
"Nothing is won in December but a lot could be lost in December," he told the Manchester Evening News.
"At the moment we are doing the right things but nothing more than that. This is my sixth season in the Premier League and every time this has been the period when you start realising which teams might go on and win it and which teams aren't maybe the real deal.
"You don't win anything but you could lose the Capital One Cup, the FA Cup, and you could lose two or three games and be out of title race."