Talk of the 28-year-old emulating Rod Laver by sweeping all four grand slam titles this year has gotten louder since he won his 16th grand slam at the Australian Open last month.
"I'll try, that's for sure," Federer said. "But it's not even number one on my to-do list. I'll just try to defend my number one position."
Story continues below ↓advertisement | your ad hereFederer said Nadal, who has been troubled with knee injuries, would be his biggest threat this year but that the Spaniard did not have the ambition to be No. 1.
"The first moment when I became number one in the world was six years ago now and it was a magical moment in my career," Federer said. "It was pretty special and I always wanted to get back there.
"Some people have that drive more than others who've been No. 1. Rafa doesn't seem like he cares as much for No. 1, or he doesn't show it."
Despite casting doubt on Nadal's motivation, the Swiss still sees him as his biggest threat. Injury forced the world number three out of the Australian Open.
"He had a good start to the season," Federer said. "What he's going through is very similar to what I went through last year. Just because you don't win a major they start picking on you. I'm convinced he will come back shortly."
The other players who could upset his shot at the grand slam this year were Novak Djokovic, Andy Murray, Juan Martin Del Potro and Nikolay Davydenko, Federer said, adding that he thought Briton Murray had "got Nadal's number a little bit".
"Davydenko has made his point and it will be interesting to see how long he can last because he's playing a lot of matches and he's not used to maybe winning that much," Federer said. "Players want to beat him even more now."
Federer was speaking to Reuters on a trip to Ethiopia to visit a school funded by his charity.
"Every season is different or special," he said. "Last year was the most emotional, the most special. Losing in Australia, coming back, winning Paris, Wimbledon, getting married, going through pregnancy, having the babies."
When asked if this season was going to be his best yet, Federer smiled: "Well, it's certainly on the right track."
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