The Saints beat Chelsea 3-1 at Stamford Bridge in their most recent outing, but Tadic doesn't believe that the result should be treated as a shock.
The Serbian international claims that Southampton's squad has the quality to come out on top regardless of who they are up against, and he has challenged the side to keep up their performance levels.
"I think just the way how we play was really important, because if we keep playing like this we can beat everyone," he said.
Southampton's win at Chelsea exposed all the problems that Jose Mourinho's side have suffered throughout the campaign so far.
The Saints battered the Blues for much of the contest, and their attacking play carved them apart time after time.
Goals from Steven Davis, Sadio Mane and Graziano Pelle provided them with the victory, and the scoreline could have been even greater, had Southampton put all their chances away.
It was a hugely encouraging performance from Koeman's side, and it has lifted the south coast up to ninth place in the Premier League table.