BERLIN: Former Swedish tennis star Mats Wilander says he doubts Switzerland's Roger Federer will ever reclaim world number one spot from Spanish rival Rafael Nadal who usurped him last month. "I do not believe Federer will be world number one again," Wilander said. "That is due to the way the ATP rankings are calculated, because you have to be powerful week after week." "If Federer wants to be world number one again, he will have to win lots of small tournaments and I don't believe that interests him." "What he is interested in doing is winning the grand slam tournaments and beating Nadal," added the Swede, who won seven grand slam titles in his career. Having won Roland Garros and Wimbledon this year, Nadal officially became world number one on August 18 this year and ended Federer's 237-week run at the top of the ATP rankings. With both Nadal and Federer having reached the US Open semi-finals in New York on Saturday, Wilander, 44, says the hard-court surface of the Arthur Ashe Stadium is the Swiss star's best chance to beat clay-court king Nadal.