Rublev has been terrible at the back end of the year too. Losing to Ruud on indoor hard, oof.Tsitsipas has phoned it in the past few months since Wimbledon. Most of his ranking points this year came early in the year when he was playing great tennis. Right now doesn't seem to have the consistency of Medvedev or Zverev hence why I rekon he is that rung below with Rublev and Berretini. Doesn't have a weakness but doesn't have a standout quality shot from what I see of him.
He was very close to beating Djokovic in the SF in New York this season, I think it'll happen but you're right it's not a certainty.Zverev has been knocking on the door for three or four years - There has been progression in the last two years in which he is regularly making it to the QF's and further - Think a slam will come BUT it's no certainty.