Novak Djokovic said he felt overwhelmed at the news he will now be the only active member of the so-called big four.

With Roger Federer and Andy Murray having already left the sport, Rafael Nadal announced on Thursday that he will also hang up his racket at the end of the season. Djokovic has spent nearly two decades battling his biggest rivals, having recorded the most grand slams of them all, 24, compared to Nadal’s 22, Federer’s 18 and Murray’s three. The Briton, of course, lost eight grand slam finals, all to one of Djokovic or Federer.

“We knew that that moment is coming sooner than later but it’s still a shock,” said Djokovic after beating Jakub Mensik to reach the semi-finals of the Shanghai Masters. “Also for Roger a few years ago when he announced retirement, and Andy as well this year. It’s a bit overwhelming for me. I don’t know what to make out of it. I still enjoy competing but part of me left with them, a big part of me.”

There appears no prospect of Djokovic following his former rivals any time soon, and the 24-time grand slam champion withstood the challenge of Czech teenager Mensik – almost 20 years his junior – to triumph 6-7 (4), 6-1, 6-4.

Djokovic looked to have the first set under control only to go completely off the boil, lose his break advantage and then the tie-break. He regrouped well at the start of the second, though, and the only real moment of alarm came midway through the third when he appeared to feel pain in his left knee.

Djokovic, who still wears a support on his right knee after surgery after the French Open, said: “It’s a strange feeling, to be honest, those couple of points in that game, but after that it was fine.

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Djokovic knows the 19-year-old Mensik well having trained with him but was determined to score another victory for the old guard. “These kind of matches, against teenagers, it’s something that really motivates me, keeps me going and pushes me to really dig deep and to show to the world that I’ve still got it in my legs, can still go the distance with the young guys,” he said. “So I’m super glad that I managed to win this very challenging match.”

In the last four, Djokovic will face the seventh seed, Taylor Fritz, who ended the run of David Goffin with a 6-3, 6-4 victory.