I think it's Mats Wilander who said that trying the things you'd do easily early in the matches at moments you feel pressure or/and are tired isn't really successful. Every pro feels pressure, then thing is to rely on basic strengths and safer plans. It's hard to go for the line when pressured at 4-5 third set 30 all serving to stay in the match.
I think it's this:
Of course he's talking about another issue, but still interesting, especially towards the end.