Celtic have the chance to cut the gap further when they take on Aberdeen at Pittodrie tomorrow lunchtime, with Rangers playing Hibernian at Ibrox on Sunday.
Asked if he thought their Old Firm rivals would "crack" in the run-in, Fortune replied: "I think so, yes.
"In the dressing room after the Hearts game, we all said that we can do it and we have to believe it.
"There are 14 games left and we have put them under pressure.
"When you see the team behind you win and win and you make some mistakes, you start to think about it and it's not good when you think about it.
"Rangers drew 1-1 with Motherwell (on Wednesday) so we see that they can lose or draw so we have to believe and I think we will do it."
Scottish Premier Winner 2009/10 Win Outright: Celtic 2/1