Jordan's argument is that he was by far the most dominant in his sport in his time. 10 scoring titles in a row, 6 rings, 6 finals MVPs, 5 regular season MVPs, DPoY
Messi's argument is that he has the greatest trophy cabinet and statistics and eye test combination in by far the most popular sport in the world.
Djokovic is absolutely amazing and probably the greatest tennis player ever, but he doesn't have an overwhelming advantage over Rafa and Roger when all three were close to their primes. I also think he (and Rafa) benefited a lot from surface homogenization. No way Djokovic should have 7 wimbledons with his style of play. But the grass isn't as fast or as low bounce anymore.
This is an all time argument. LeBron is in the GOAT debate.
In Tennis, the 3 GOATs have coincided in the same era. The most clutch is Novak. Doesn't matter who the greatest is. We're just talking about who is the most clutch. It's just one aspect of greatness. I'm literally a Federer fanboy myself.
Messi is arguably the GOAT but doesn't even make it into top 10 most clutch Footballers ever. Messi was not known as an especially clutch player for big parts of his career.
So basically it comes down to MJ vs Nole in this category. I think Nole takes it. Individual sport requires more mental toughness and bottle in the crucial moments. You can't rely on anyone else for a single second. You're not allowed a single weak moment during a deciding point. It's relentless. If you've played both Tennis and Footy at a decent level, you'd know Tennis takes way more mentality than Football.
Jordan/Federer/Ali might be greater but Djoker is more clutch.