Fonseca feels indeed like a blend of Alcaraz and Sinner, a new mix of qualities. A precocious player, less athletic than Carlos, shorter than Jannik, but surprisingly complete. His body reminds me of Federer and he slices well.
Some of his points had a Sinnesque quality of suffocating the lighter opposition, combining great movement with heavy weight from both wings and being able to finish points with screamers. However he was attacking much more, similar to Alcaraz and doing well at the net. First data indicates that he will do great in longer rallies, a mark of quality. This doesn't mean that he will be better than one or the other, but he has an excellent base to become a great player on all surfaces.
He might well lose against crafty Tien, but we have a star here.