Novak Djokovic "feels amazing" after surpassing Pete Sampras for the year-end number one record on the ATP Tour.

Djokovic was presented with the trophy for finishing the year ranked number one for the seventh time after defeating Casper Ruud 7-6 (7-4) 6-2 in his ATP Finals opener on Monday, breaking a tie with idol Sampras.

A 20-time grand slam champion – a joint record held alongside Rafael Nadal and Roger Federer, Djokovic revelled in his latest achievement.

"It feels amazing and it feels even better when you win a match and then get your hands on the trophy that I have been blessed to lift seven times," Djokovic said in Turin.

"[I have won] one more than Pete Sampras, who was my childhood hero. He was the one that got me into tennis. He was also an inspiration to me and I dreamt of being a Wimbledon champion and World No. 1 like he was.

"Fast forwarding to today, it is quite amazing to be in this position. I am very grateful. It is something I am very much appreciating and not taking for granted."

World number one Djokovic is looking for his sixth ATP Finals title, though his last success came in 2016.

Djokovic produced a confident performance in his opening match against Ruud, dropping just four points behind his first serve and rallying from a break down in a tight opening set.

The top seed – who tied Ivan Lendl for second-most wins (39) in the history of the ATP Finals – will also face Stefanos Tsitsipas and Andrey Rublev in the Green Group.

"It was a really terrible start, but also funny because I’m still trying to figure out what happened," Djokovic said. "Casper started strong. He was serving well. The altitude, fast court, fast balls – it favours big servers. I knew he had a solid serve, but maybe not as good as Medvedev or Zverev.

"He did positively surprise me with this serve, particularly in the first set. I just managed to read it better in the second set. But it was a close one."

Djokovic, meanwhile, expressed his shock amid the unknown whereabouts of WTA player Peng Shuai.

The Women's Tennis Association (WTA) has opened an investigation into Peng's sexual assault allegations against a former China leader.

Shuai, 35, posted on Chinese social media site Weibo allegations against Zhang Gaoli – the ex-vice premier and member of the Chinese Communist Party's Politburo Standing Committee – claiming he had forced her to have sexual relations with him.

In the post, published on Tuesday before subsequently being deleted, the 2013 Wimbledon doubles champion alleged that the pair had extramarital relations and she had developed feelings for him.

All of Peng's content has since been removed from Weibo and numerous reports suggest she has not been seen in recent days.

"I did hear about it a week ago. Honestly, it's shocking that she's missing, more so that it's someone that I have seen on the tour in the previous years quite a few times," said Djokovic.

"It's not much more to say than hope that she will be found, that she's okay. It's terrible … I can imagine just how her family feels that she's missing."