US Open 2019 news - Roger Federer tumbles out in shock loss to world No. 78 Grigor Dimitrov

Coming into the U.S. 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-times Grand Slam winner.

"Even if I would have gone two sets to love down, I wouldn't have give up," assured Dimitrov. "I would still stay on the court and just try to do as much as possible to make sure that whether I rattle him or put him off balance.

"I kept on pushing, I kept on believing."

Federer looked razor sharp to start the match but was not able to maintain his usual high level against the gritty Bulgarian.

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.

Dimitrov will now face Daniil Medvedev on Friday for a place in final, the Russian advancing to the last four with a 7-6(6) 6-3 3-6 6-1 win over another Swiss Stan Wawrinka.

While not as crushing as his five-set loss to Novak Djokovic in the Wimbledon final, is was another bitterly disappointing missed opportunity for Federer to add to his Grand Slam haul.

After a sputtering start to the U.S. Open that saw him drop the first set in his opening two matches in astonishingly submissive style, Federer had looked more like the silkly smooth Swiss maestro fans have come to expect.

He was virtually flawless in his next two contests, dropping just nine games total in straight sets wins setting the tennis world buzzing about a possible Grand Slam Big Apple final showdown with old rival Rafa Nadal.

"Just disappointed it's over because I did feel like I was actually playing really well after a couple of rocky starts," said Federer. "It's just a missed opportunity to some extent that you're in the lead, you can get through, you have two days off after.

"It was looking good.

"But you got to take the losses. They're part of the game."