'A part of me left with Roger, Rafa and Andy': Djokovic

Novak Djokovic says he feels overwhelmed at the news that he'll be the only member of the 'big four' of men's tennis left playing.

With Roger Federer and Andy Murray having already left the sport, Rafael Nadal announced on Thursday that he will also hang up his racquet at the end of the season.

"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."

Still, there's no prospect of Djokovic following his former rivals any time soon, as the 24-time grand slam champion withstood the challenge of Czech teenager Mensik - almost 20 years his junior - to triumph 6-7 (4-7) 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 following 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.

"So, hopefully, when it cools down I'm not going to have any troubles with it, because I already have troubles with the right knee, so it wouldn't be great."

Djokovic knows 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 seventh seed Taylor Fritz, who ended the run of David Goffin with a 6-3 6-4 victory.