Grigor Dimitrov came from two sets to one down to claim the most memorable win of his career and end Roger Federer's hopes of a 21st grand slam title in the US Open quarter-finals.
Once nicknamed 'Baby Fed', Dimitrov has failed to live up to the significant promise he displayed earlier in his career, unable to build on his run to the Wimbledon semi-finals in 2014.
He reached the same stage at the Australian Open in 2017 but has since found success at grand slams hard to come by, and his progression to the last eight at Flushing Meadows marked his first tour-level quarter-final for eight months.
The world number 78 was viewed as likely to be overmatched by the 38-year-old Federer, who had breezed through the first four rounds of a draw that defending champion Novak Djokovic tumbled out of in the last 16.
However, after Federer took the first set, Dimitrov displayed incredible character and unleashed the full repertoire of strokes that led to the comparisons with the Swiss legend.
When he squared the match to effectively turn it into a three-set contest, it was the Bulgarian who had the greater endurance, Federer taking a medical timeout at the end of the fourth set.
Federer was never the same player after he re-emerged for the decider and Dimitrov took control of the fifth to complete a 3-6 6-4 3-6 6-4 6-2 triumph in three hours, 12 minutes and book a fascinating semi-final with an ailing Daniil Medvedev.