Dominic Thiem is of the opinion that grand slam trophies should be the defining criteria for deciding which player is or should be the greatest of all time.
In that regard, the race is currently tied between Novak Djokovic and Rafael Nadal. Thanks to his heroics in Melbourne, Djokovic equalled Nadal at 22 right now with the race making the upcoming Roland Garros even more exciting. Thiem played in grand slam finals against both players and he knows how hard it is to get one having lost multiple before winning the US Open.
It's key for him in the GOAT debate:
“In my opinion, the Grand Slam titles should be the defining criteria when determining the best of all time, they are the four most important tournaments in tennis. Everything else is fine, but it’s not the same. The Slams are what counts, so the GOAT will probably be the one with the most Grand Slams.”
Thiem also added that he wasn't surprised to see Djokovic win:
I am not very surprised, Djokovic still looks young. Physically and mentally, because of the way he moves on the court. It’s like he was 25 years old.“We have to be honest, he is the best, so his victory was not very surprising.”