Not so sure tbh I think some of these matchups might be closer than you think.
Medvedev vs Hewitt - Hewitt on natural surfaces, Med on HCs
Thiem vs Safin - yeah Safin is better anywhere off clay. Tim is more consistent but Safin's peak is godly
Zverev vs Roddick - Roddick on grass, Zed on clay, 50/50 on HCs
Stefanos vs Ferrero - Ferrero slightly better on grass and clay, but Steffy is probably better on HCs
Berrettini vs Nalbandian - Berry on grass, Nalby on clay, and 50/50 on HCs (Nalby underachieved in BO5 HCs)
Alcaraz vs Davydenko - I think Carlos is clearly a better player than Davy
Sinner vs Gonzalez - I think Gonzo has the advantage in this case except on grass
Ruud vs Ferrer - Ferrer is clearly superior everywhere
FAA vs Blake - I think Felix has an equally slight edge on each surface
Shapo vs Baghdatis - this one is gonna be a slug-fest
Kyrgios vs Philippoussis - 50/50 although this would be a grudge match for both players
Rublev vs Ljubicic - Rublev slightly better on each surface but not by a lot
You’re giving a lot of credit to the younger guys. Let's split it up by surface for each of them:
Medvedev vs. Hewitt: Hewitt clearly on clay and by a massive margin on grass. I would give Hewitt hard court too, peak for peak, but let's be generous here and hand hards to Med. Even so, that's by a tiny margin and surely not enough to outweigh the massive lead Hewitt has on natural surfaces.
Thiem vs. Safin: Safin unquestionably on hard and even grass. Clay goes to Thiem but Safin wasn't terrible there.
Zverev vs. Roddick: Roddick clearly on grass, Zverev clearly on clay. I actually respect Zverev's peak level on hard as he's come up with some good stuff there but I would still hand it to Roddick here because unlike Zverev, Roddick actually brought that peak level to the hard court Slams. Z probably gets best of 3 hard though.
Stef vs. Ferrero: Ferrero on both grass and clay; peak for peak I would actually take hard court too as I've never been too impressed by Stef on hard courts; his best is reserved for clay, and even there Ferrero was better.
Berrettini vs. Nalbandian: I think Nalby is very overrated but I would still take him over Berrettini here. Berrettini gets the edge on grass and Nalby on clay, neither by significant margins, and hard court I would lean Nalbandian. He did underachieve at the Slams but his underachieving is still better than Berrettini's results. The match with Roddick at the 2003 US Open and the match with Federer at the 2004 AO are still firmly above anything Berrettini has done there IMO. When we bring Bo3 into the conversation it's even more lopsided as we can bring his 2007 indoor swing into the mix.
Alcaraz vs. Davydenko: I have no doubt that Alcaraz will become the better player as time goes on (he's not even really in the Zverev/Med/Tsitsipas gen), but for now I think I'd hand it to Davydenko. Alcaraz possibly gets clay (it's close, Davydenko could definitely play on the surface as his Rome 2007 match with Nadal showed) but Davydenko was firmly better on hard courts peak-to-peak. Grass is basically a wash. Again, Carlos hasn't even hit his peak yet (hopefully) so I expect him to get even better but as it stands Davydenko has shown a higher level of tennis on the whole.
Sinner vs. Gonzalez: I also think Sinner has still yet to peak, but I'd take Gonzalez most places as of now. Except for maybe grass.
FAA vs. Blake: I'd take FAA everywhere except hard courts. I also believe he's still got time to improve.
Shapo vs. Baghdatis: I might actually give this to Baghdatis everywhere lol.
Kyrgios vs. Philippoussis: Scud wasn't really from Fed's gen and he was only there for one Slam final but whatever. I think Scud wins everywhere but by close margins.
Rublev vs. Ljubicic: Both players have been extreme underachievers at the Slams so they don't tell us a whole lot. I think Rublev is better everywhere but hard courts (and especially indoors).
