I don't follow the OP's point that "no lead is safe" in a five set match. That would certainly also be true in a three set match, moreso actually in a three set match because being up 2-0 in sets in a five setter is more likely to demoralize the opponent (i.e. the lead is safer) than being up 1-0 in a three setter where the opponent realizes he doesn't have such a high mountain to climb.
"Where's the REST of me?"
Ronald Reagan, in "Kings Row"