Sinner started the match as the heavy favourite after a week of serene progress.
The 23-year-old had dropped just 17 games and spent only five hours and 23 minutes on court – less time than the entirety of his epic French Open final against Carlos Alcaraz – on his way to reaching the fourth round.
But it was a different story against the experienced Dimitrov – the first seeded player Sinner has come up against so far.
Dimitrov was alert, adaptable and quick, returning everything Sinner threw at him in a dominant display, and a break of serve at the first opportunity stunned a crowd expecting to watch another demolition job.
It proved to be decisive as Dimitrov produced a magnificent serving display, saving the sole break point he faced before closing out the first set.
Sinner looked nervy and, with Dimitrov refusing to let his level dip as he went an immediate break up, he struggled to find any momentum to get back into the contest.