By Pritha Sarkar

LONDON (Reuters) – No one has come close to cracking the riddle of beating Novak Djokovic on an indoor court for more than three years and Kei Nishikori became the latest player on Sunday to discover just how difficult it is to solve that puzzle.

Djokovic began his bid for a record fourth successive title at the ATP World Tour Finals with a brutal yet awe-inspiring 6-1 6-1 hammering that left Nishikori red faced.

“No question, it felt like the best tennis I played this year,” the 28-year-old Serb told reporters.

“I was at my best and it was an incredible performance.”

Before the start of the match, both players stood heads bowed during a minute’s silence in memory of the victims of Friday’s Paris attacks — with the French flag’s red-white-blue tricolour plastered on the electronic banner surrounding the O2 Arena.

It was not long before Djokovic was at his unrelenting best.

The Serbian world number one showed the kind of form that has made him an unbeatable force on indoor courts for more than three years with a 65-minute demolition job.

Djokovic subjected Nishikori to an array of heavy-duty groundstrokes, cruel lobs and sublime volleys as he chalked up his 38th successive win on an indoor court dating back to October 2012.

As Djokovic romped to a 3-0 lead in the opening set, there were few opportunities for Nishikori’s fan club to pick up the vast numbers of Japanese flags draped over the arena’s metal railings.

The red and white flags made a fluttering appearance when Nishikori finally held in the fourth game but the joy was fleeting as Djokovic easily sealed the first set before finishing off his opponent with a backhand volley.

“He played unbelievable tennis. I’m very ashamed with this score,” said Nishikori, who is hoping for a better showing in his next two round robin matches.

“I couldn’t do anything… didn’t feel like I had any chances today.”

Since none of Djokovic’s rivals have worked out how to beat him indoors, one reporter asked the Serb for a solution.

“If I had (a solution), I wouldn’t share it with you, that’s for sure,” he quipped.

Djokovic, who has lost only five times during a stellar season in which he won three out of the four grand slams, still has to play round-robin matches against Roger Federer and Tomas Berdych in the Stan Smith Group before the top two players advance to next weekend’s semi-finals.

The season-ending tournament features the world’s eight best players who are split into two groups. Andy Murray, Rafa Nadal, French Open champion Stan Wawrinka and David Ferrer will begin battle in the Illie Nastase Group on Monday.

(Reporting by Pritha Sarkar, editing by Toby Davis)