Djokovic was coming back after an operation with very limited clay preparation for RG, also he was mentally really tentative in that match, as he really regained his clutchness and confidence only at Wimbledon that year.
Cecchinato maxed out his potential that year with a favorable draw, after that he won 2 minor clay titles, but recently even Monteiro defeated him in a clay Challenger final.
That's a big downfall and totally signifies his status now: a has-been who cannot even bring results on his favorite surface anymore...