SPL - Fletcher expects 'one of those nights'
Craig Levein's men are one point ahead of Czech Republic going into the last set of fixtures and a win against the world and European champions would guarantee them second place.
However, the Scotland skipper said: "To speak about getting a point, although it might be a great result, it might be irrelevant, it might not be enough. We know going into the Spain game that it is in our hands. But we might lose and be in second place and we might draw and not be in second place so it is going to be one of those nights."
Fletcher added: "We can only try to do our best and we will give it everything and although people are maybe writing us off, while there is still a chance we will keep believing."
The Manchester United midfielder, who is confident of shaking off an ankle knock picked up in the 1-0 win over Liechtenstein in Vaduz on Saturday night, reiterated the enormity of the task awaiting the Scots.
"We are under no illusions," he said.
"There is no question that Spain are the best side we've played against, they are European and world champions and they have not been beaten in a long time. They will have the majority of possession.
"I watched them against the Czech Republic the other day and they had 65-70% of the ball. So we have to go with a gameplan. If we open up too much and try to take the game to Spain then I think we would get punished and be out of the game fairly quickly. So I think in that respect it is going to be difficult.
"The biggest thing is that we won the match against Liechtenstein the other night which put us into second place, and it puts the pressure on the Czech Republic.
"Hopefully Lithuania do us a favour as well which would be nice."