I just watched the Diallo match and was thoroughly impressed. He's into the 3rd round after beating the 24th seed in 4 sets.
I'm gonna say right now that he will have a better career and ranking than either Felix or Shap achieved. He knows how to work points unlike Denis the headcase and seems to have ice in his veins unlike Felix the choke. He seems calm and composed. Even in the post game interview, he barely smiled. Very business like.
A big thing that he has going for him is his build. 6'8" and sinewy. He moves very well for a big man and with a big wing span and can cover a lot of the court and the net. He comes to the net at very opportune times during a point (didn't see a lot of serve and volley) and is a good volleyer when he did come in. As well, with his height he's hard to lob over. It also gives him a big advantage serving both in terms of speed and being able to come down steeper. As he gains experience and muscle he will be a force to be reckoned with.
Barring injury, he'll be AT LEAST top 5 within 2 years and unlike the aforementioned other 2, will maintain a lofty ranking.
He plays Tommy Paul next round. No predictions for that match because this is all very new for him. He could be the breakout star of this Open, but he'll have to beat Paul. If he does, everybody will be talking about him.
Or I could be totally wrong and he'll just be another bum from Canada.