Roger Federer admitted he "couldn't be more happy" after he defeated arch-rival Novak Djokovic to qualify for the last four at ATP Finals 2019 at the O2 Arena on Friday. 

Having lost a Round-Robin match in the Bjorn Borg Group, it was a must-win fixture for both the former champions. Third-seeded Federer clinched a dominating 6-4, 6-3 victory and the Serbian's loss meant, Rafael Nadal has been guaranteed to finish as the year-end No. 1 ATP player. 

The 38-year-old Swiss Master had not won against Djokovic in four years but he put an end to the four-match losing streak in style in London as he secured his 16th semi-final in 17 appearances at the ATP Finals. 

Going into the match, a lot of questions regarding his poor streak against Djokovic were directed to Federer, who was clever enough to give it back after his superb win.

"[The ghosts] were never really there. There [were] some press guys that made that up. So from my standpoint, I also didn't know I hadn't beaten him in a few years. Didn't feel that way because it was so close in Paris [2018] and Wimbledon against him."

"I felt like I was going to have my chance, to be quite honest... I'm just happy at the level I could play today, and obviously, it's always special beating Novak, even more so [because] of what happened. But I didn't feel like I had to get rid of the ghosts or anything like that. I feel like I moved on pretty quickly after that," Federer, who could face Nadal or Stefanos Tsitsipas in the semi-final. 

When he was asked what he did differently at the O2, Federer told ATP, "I won match point, I guess. It was so close to Wimbledon. It was a privilege to play that match, so many ups and downs. I couldn't be more happy right now. I think I served great, had great anticipation, clear game-plan and it worked great tonight - hopefully not for the last time against Novak."

Talking more about his game against Djokovic, Federer added, "I knew the beginning wasn't key, but I knew it was important. I had a great run on the serve, as well, throughout I think that first set. I was able to keep the pressure going and mix up my game. Because he was playing incredibly aggressive at one point midway through the first set, which surprised me a little bit. Things just worked very well for me. Tonight was one of those nights where I was clear in the game plan. I got what I kind of expected, and it was a great feeling at the very end."