Beijing: World No.1 Novak Djokovic and Serena Williams proved they are still in a class of their own in the tennis world, winning six of eight Grand Slam titles of 2015. 

Djokovic collected 11 titles and 82 wins which brought him three of the four majors -- Australian and US Opens and Wimbledon, reports Xinhua. 

"It's the best year of my life, no question about it. Everything is working great. I'm very grateful for the opportunity to be able to play this well, to be successful. I don't want to get carried away by success because I want to be playing on this level for many more years to come," the 28-year-old said in October. 

Asked about his secret to success, Djokovic said it's all about self-discipline. 

"I take care of my body. I respect the preparation and recovery routines that I have with my team. It's all about self-discipline and being able to understand what you need to do to have the kind of longevity and to endure throughout the entire year, to be able to play this many matches on a high level," said the Serb, who now has 10 Grand Slam titles. 

"I know that one of the things that you have to do in order to keep playing on a high level is stay committed and stay determined to this lifestyle that is not easy, travelling all the time. Now that I'm a father, that makes it a little bit more difficult. I have the support of the close ones. Everybody is behind me on the quest of being the best I can be." 

Djokovic also swept away six of the nine Masters and made it perfect with a fourth successive season-ending World Tour Finals triumph in London. He reached the final of every tournament he played with the exception in Doha where Ivo Karlovic stunned him in the quarters. 

From then on it was one-way traffic -- indoors, outdoors, hard court, clay and grass -- the Australian Open, Masters in Indian Wells, Miami, Monte Carlo and Rome and then Wimbledon, US Open, Beijing, Shanghai and Paris Masters and London. 

But unfortunately, Djokovic lost to Swiss Stan Wawrinka at Roland Garros final which left him still seeking a first French Open and a career Grand Slam. 

Roger Federer, who hasn't added to his 17 majors since 2012 Wimbledon, continued to play his top game. Despite passing 34 in August, the Swiss reached the Wimbledon and US Open finals and handed Djokovic a rare defeat in the round-robin section at the World Tour Finals before the Serb took revenge in the title match. 

The Swiss ended the year at No.3 and decided to skip four straight Masters next season as he points his campaign towards the Rio Games and also saves more energy on Grand Slams. 

Scot Andy Murray finished No.2 in a season where he led Britain to a first Davis Cup title in 79 years. 

While 14-time Grand Slam winner Rafael Nadal has been struggling all through the season but he believed he can come back on the right track. 

"I always have the feeling that I am going to play better. I believe my level is to be there in the top eight. So even if I've had tough moments this year, I believe I can come back into the game," the Spaniard said. 

In women's tennis, the case was similar as Williams also won three of the four Grand Slams -- Australian and French Opens and Wimbledon. 

Williams looked almost certain to become the first player since Steffi Graf in 1988, and just sixth player overall, to complete a calendar Grand Slam after reaching the US Open semis. With Flavia Pennetta then dumping out second seed Simona Halep in the first semifinal, the odds on the American landing a historic title shortened further. 

Riding a 26-match win streak at Flushing Meadows, the World No.1 was overwhelmingly expected to sweep aside unseeded Roberta Vinci and set up a showdown with eventual champion Pennetta, against whom Williams owned a 7-0 career edge. 

Instead her bid was crushed in stunning fashion as the 32-year-old Vinci rallied from a set down to claim a sensational triumph, beating Williams for the first time to set up the first all-Italian women's Grand Slam final in the Open Era. 

Despite the defeat, Williams is the one that called the shots in 2015 -- like always -- and her supremacy will be continuing next season.