Novak Djokovic said a part of him had "left" after Rafael Nadal followed his other great rivals Roger Federer and Andy Murray in announcing his retirement.

Djokovic enjoyed a long-lasting rivalry with Nadal, along with Federer and Murray, with tennis' 'Big Four' winning a combined 69 grand slam titles.

The Serbian is currently aiming to join Federer and Jimmy Connors as the only players to achieve 100 ATP Tour-level titles, and is two games away from doing so at the Shanghai Masters.

Djokovic overcame a plucky display from Czech teenager Jakub Mensik on Friday, coming from a set down to win 6-7 (4-7) 6-1 6-4 and progress to a record 77th semi-final at Masters 1000 events.

The triumph came a day after Nadal confirmed he would retire next month, following 60 head-to-head meetings with Djokovic, the latest coming at the Paris Olympics earlier this year.

Djokovic won 31 of those encounters, with four coming in grand slam finals at the US Open, Wimbledon and the Australian Open.

With Federer announcing his retirement in 2022 and Murray calling time on his career after the Olympics, Djokovic will soon be the only active player from the 'Big Four' that dominated the sport for many years.

And while the Serbian is still enjoying his tennis, he got emotional at the "overwhelming" feeling of being the last man standing.

"He has been my greatest rival and my rivalry with him has impacted me the most in my career by far," Djokovic said.

"We knew that moment is coming sooner than later but it's still a shock.

"When it came officially for Roger a few years ago as well, when he announced retirement and Andy as well this year. It's overwhelming for me to be honest.

"I don't know what to make of it. I still enjoy competing but part of me left with them. A big part of me. It's tough news for the tennis world, the sports world.

"Rafa is an inspiration to millions of children around the world so he can be very proud of his career."