(CNN) -- Rafael Nadal is hoping to bounce back from his Shanghai shock by ending the 2011 tennis season on a high note.
The Spaniard has lost his No. 1 ranking to Novak Djokovic despite winning the French Open for the sixth time in June, and has taken a month off after his surprise defeat to 23rd-ranked German Florian Mayer in the third round in China.
Nadal has never won the season-ending ATP World Tour Finals, where he opens his Group B campaign against Mardy Fish on Sunday in London. But the 25-year-old said he feels confident ahead of the $5 million tournament and next month's Davis Cup final against Argentina.
"To finish the season with a good feeling, like I had last year, is important. It's positive, and I'm working for that," said Nadal, beaten by Roger Federer in the 2011 London final.
"I am feeling good. I feel happy about how I am practicing and doing things. I'm happy about how my mind is working, and I'm working hard every day with enough motivation. That's the most important thing.
"To lose Shanghai was difficult to manage because I felt I was in a positive moment. I felt I had an opportunity to play a good tournament, and I had a bad loss against Mayer, so that hurt me a little bit.
"I felt that I needed to stop, to practice a little bit, to recover a little bit physically and mentally, and that's what I did. Not only thinking about this year, but thinking about trying to finish the season healthy and try to start next season with positive conditions."
Federer is seeking a record sixth title, coming into the indoor hard-court event on a 12-match winning run after his triumphs in Basel and Paris.
The 30-year-old, a 16-time grand slam champion, will launch the tournament in his Group B opener against France's Jo-Wilfried Tsonga before Nadal takes to the court.
"This year will be very interesting because I feel the one to four guys are the favorites, and then from five to eight they have the potential to upset the top guys and go all the way to the final," said the Swiss, who beat Tsonga in the Paris Masters final last weekend.
"They have big games, they can beat anyone on any given day and they are good enough to win this too. They are very even groups. All the top eight guys have the potential to do anything here."