Put a roof on a building and Novak Djokovic is unstoppable. Admittedly, he is not too bad on other surfaces as well. Thirty times opponents have tried to beat him indoors since October 2012, 30 times they have failed.

You have to go all the way back to 31 October, 2012 for a second-round match of the Paris Masters for the Serbian's last indoor loss. Sam Querrey won 0-6 7-6 (7-5) 6-4.

Interestingly, Djokovic walked into the arena in Paris wearing a Darth Vader mask. The Force was certainly not with him though.

Image source, AFP

Image source, AFP

The man trying to stop Djokovic today is Japan's Kei Nishikori.

The 24-year-old became the first Asian player to qualify for the ATP World Tour Finals after reaching the US Open final in September, beating Djokovic in the semi-finals before losing to Marin Cilic in the final.

Will Nishikori be jumping for joy today? Stay with us. We will soon find out.

Hello, good afternoon and welcome to BBC Sport's live text commentary of the first semi-final from the ATP World Tour Finals.

Novak Djokovic was the winner at the O2 in London in 2012 and 2013. No one has won the season-ending competition in three successive years since Ivan Lendl's successes in 1985, 1986 and 1987.

He has already secured the trophy for ending 2014 on top of the world rankings, and he may soon be looking to find more space in a very large and very packed trophy cabinet.

Image source, AFP

Novak Djokovic has not lost an indoor match for more than two years and he has been in stunning form in London so far this week as he cruised into the semi-finals of the ATP World Tour Finals.

Image source, AFP

Six sets played, six sets won. Who can stop him from winning the title for a third successive year?