ATP Tour Finals: Federer content for Nadal to be favourite Date: 10 November Coverage: 14:00 GMT on BBC Two and BBC Sport website Roger Federer is happy to be the underdog going into Sunday's semi-final against Rafael Nadal at the ATP World Tour Finals in London. The Swiss struggled with a back injury earlier this year and has slipped to seventh in the world rankings. "I need to look at it more as being an underdog a little bit because of circumstances, because of my year, because of his year," said Federer, 32. Maybe that free swinging is what I need to do a little bit more. Federer added: "We know each other that well. He's going to do exactly what he needs to do, I'm going to try to do what I need to do. Hopefully it's going to match up good for me. Pumped-up Federer sees off Del Potro If not, he deserves it. He's had a wonderful season. We'll see how it goes. World number one Nadal has won all three meetings between the pair since he came back from a serious injury in February, and Federer's last win over the Spaniard was at Indian Wells in March 2012. "I don't know if the goal was necessarily for me to try to win this event right off the first match," said six-time champion Federer. I knew I had so much work to do with Djokovic in the first round. I was curious to see how I was going to adapt, how everybody was going to adapt, to the London conditions. For that reason I'm very pleased now. I feel very happy and very upbeat. I'll give it one more big shot and hopefully I'll make it to the finals. Nadal, 27, secured his semi-final place on Friday and is looking to win the season-ending title for the first time. "I am in the semi-finals without losing a match, so that's the most important thing for me," he said. The only way to win the semi-finals is playing my best match, playing fantastic tennis. Following on from Nadal's match with Federer at 14:00 GMT will be world number two Novak Djokovic who takes on seventh seed Stanislas Wawrinka in the second semi-final.