World number one Novak Djokovic will bid for a record fourth successive ATP World Tour Finals crown in London later this year after becoming the first man to qualify for the season-ender.Five-times Australian Open champion Djokovic reached the French Open semi-finals by crushing Rafa Nadal on Wednesday and has been virtually unbeatable this season and is currently on a 27-match unbeaten run.Serbian 28-year-old Djokovic is looking to become the eighth man in history to complete a career grand slam by winning the French Open this week.Djokovic won the elite ATP Tour Finals for the first time in Shanghai in 2008 and has a hat-trick of titles in London, last year gaining a walkover against injured Roger Federer."I had a great start to 2015 so I am really happy to have qualified so early. I hope I will have a great finish to the year too; I love playing at The O2," Djokovic said.