The outspoken and often philosophical Janko Tipsarevic described the incredible run of his fellow Serbian Novak Djokovic as a “joke” ahead of their quarter-final match-up on Wednesday at the US Open.

Tipsarevic, seeded 20th, is playing the tennis of his life but the 27-year-old remains in awe of his good friend’s accomplishments this year.

“It’s really a joke. I cannot believe that somebody lost two matches this year," said Tipsarevic. "You see guys on tour, and I’m telling you this from a psychological point of view, they win a couple of matches in a row, and you could feel in their body language on court that they feel a little bit satisfied.

“You really need to be big, like a big, big champion in order to really do what you say in press conference.”

While Tipsarevic said that facing a good friend on court has both its advantages and disadvantages, Djokovic admitted it was not a very comfortable feeling.

“It’s a strange feeling. We are professionals. Certainly we both want to win the match when we play against each other. So you kind of forget about friendship,” he said.