Coming into the US Open, the 78th ranked Dimitrov, dubbed "Baby Fed" because of the similarities between his game and Federer's, did not look much of a threat, posting just one win from his last eight matches.
But the Bulgarian, who had been ranked as high as number three, has found some form on the New York hardcourts and claimed his first win in eight meetings with the 20-time Grand Slam winner.
The turning point came in the fourth set, Dimitrov fighting off five break points to win the set and level match at 2-2.
Before the start of the fifth set Federer called for the trainer, who, after a brief conversation with the 38-year-old, escorted him off the court for a medical time out to deal with an apparent back problem.
When play resumed Dimitrov seized his chance, breaking Federer to open the fifth and then a second time with the Swiss unable to mount any defence as the Bulgarian went on to claim the biggest win of his career.
