England need a win over Slovenia in Port Elizabeth on Wednesday to assure themselves of a place in the last 16 in South Africa after drawing their opening two games.
For Slovenia's part, they need just a point to go through and were eight minutes away from reaching the knockout stages until USA pegged them back to 2-2.
Auxerre playmaker Birsa believes his team need to be stronger mentally against England than they were against the US, where they were well on top until Landon Donovan scored early in the second half to bring the Americans back into the game.
"We learned to make sure we don't finish before the game finishes," said Birsa. We must not allow ourselves to get it into our head that we have won the game. You have to be more clinical and wait for the possibility to score.
"In the first half we played well and at 2-0 the Americans changed their system. It should not have been so difficult but in our heads everything was going down, down, because they scored very quickly and that made it very difficult for us.
"All the time you must try to play better. England are the most difficult team in this group, but you have seen already at this World Cup that the small teams can win against the big teams, so we know we have a possibility."