Defending ATP World Tour Finals champion Roger Federer on Friday said the field is wide open at the 2011 year-end championship, which begins at London\'s O2 Arena on November 20. The final eight was determined by results at this week\'s Paris Masters, with Tomas Berdych, Jo-Wilfried Tsonga and Mardy Fish joining Novak Djokovic, Rafael Nadal, Andy Murray, Federer and David Ferrer. \"I don\'t feel like 5, 6, 7, and 8 have no chance,\" commented the Swiss after claiming his 800th tour-level victory with a quarterfinal win over Juan Monaco on Friday in Paris. \"I definitely think they have a chance to go deep, like it was in the year with del Potro and Davydenko making the finals. I definitely could see something happening as well like that this year. \"I definitely think Fish and Djokovic at this point look a little bit weary. But then again, I think with a week\'s break everything can heal and they\'re maybe going to be in top shape. So let\'s assume that everybody is in great shape. I think it\'s going to be an interesting couple of groups, because everybody can beat everybody I think again.\" The 2010 edition of the world\'s biggest indoor tennis tournament saw the Big Four of Nadal, Federer, Djokovic and Murray coming out on top in the group stages to set blockbuster semifinal clashes. It was Federer who ultimately prevailed, defeating Djokovic and then Nadal to win the season finale for the fifth time. \"I do believe if Djokovic is fit, and Murray is fit as well, which it seems like he is, they\'re also going to be difficult to beat,\" continued Federer. \"And Rafa, regardless how he is going to be, he\'s always a tough customer. So I think it\'s going to be a really good World Championships. I\'m really looking forward to it. It\'s a strong year, and I\'m happy to be part of it.\"