Shanghai - Roger Federer on Saturday admitted he hated being described as the world number two and said he was desperate to win back the top ranking from Rafael Nadal.

Federer said being introduced as the world's second best player was the worst thing about the end of his record four-and-a-half-year reign in August.

"It doesn't change a whole lot, I just don't like the ring of it when I'm being introduced on Centre Court saying, 'and this is the number two in the world,'" he said.

"It just sounds wrong. Either I'm number one or I'm a Grand Slam champion, but I'm not number two."

The 27-year-old Swiss said he was using his anger at the "number two" tag as motivation to win back the top spot next year.

"It just sounds a bit awkward for me because I've been up there for so, so long," he said.

"It sounds unfamiliar but at the same time it is a challenge to get back where I was and I think next year will be a good chance. So I'm going to try to do everything to do that, starting here."

Two-time defending champion Federer can gain useful rankings points on Nadal at the Masters Cup, which starts Sunday.

Spain's Nadal, who is already assured of finishing the year as number one, is skipping the year-end showpiece citing fatigue and tendonitis.

Federer has reached five finals in seven Masters Cup appearances, losing just three matches in total.

He is bidding to join Ivan Lendl and Pete Sampras as only the third man to win five year-end titles, and opens his campaign against Britain's Andy Murray on Monday.