one word: power.
Fed just isnt as explosive as he used to be, and even at his best, never hit hs bh as ferociously as stan does. And his power, has diminished somewhat over the years.
but lets get one thing straight. styles make fights. matchups matter.
Stan is the fittest and strongest Ive ever seen him. The best way to beat defensive retrievers is just to blow them off court. This is what fed realized when he turned his h2h around with Hewitt. He just started beings super aggressive and attacking hewitt and the rest is history. See USO 2004.
Hence, right now, Stan with his improved fitness and explosie power is a bad matchup for both Rafa and Nole right now.
However, Fed still does ok against djoker and murray..but against Rafa he is hopeless. why?. Because for all stan's strength and power, his game is not the most varied. Hit the fh hard, hit the bh harder.
Meanwhile fed has all manner of spins, angles and shots he can deploy, and that still buys him purchase against every player on tour not named nadal.
Against Nadal, all his variety is worthless because Nadal reacts so quickly to the ball, retrieves it, and crushes it and at fed's age, its a tough ask to dig those shots out all day long.
. Aside from endless grinding( djoker) the only thing that consistently beats nadal is pure raw, well placed power. (delpo, soderling, rosol, berdych, blake back in the day). Guys who are fit, can hang in the rallies, and consistently blast the ball off both wings can beat Nadal. Cue Stan 2.0
That said, if fed this year on a good day played Wawa, he'd have a chance because his variety would keep wawa on his toes and at least keep him guessing. Its easier I think to get in a rythym against nadal because you know what kind of ball you will get 90 percent of the time.