INDIAN WELLS (California) (AFP) - Confidence and concentration make world number one Rafael Nadal a formidable foe, but they don't make the world number one unbeatable, rival Novak Djokovic said Friday. Serbia's Djokovic, ranked number three in the world, defeated Nadal in the semi-finals en route to the title here last year, and he could face the same task again if he is to defend his crown. This time around, however, Nadal arrives in the California desert as the world number one, having toppled Swiss star Roger Federer from the top spot last August.He also has the first Grand Slam title of the year to his credit - after a thrilling triumph over Federer in the Australian Open final.Despite some nagging troubles with his knee, Nadal is a force to be reckoned with, Djokovic said."Well, we're coming back again to this confidence," Djokovic said. "It's incredible... when you start winning a lot of matches then you just feel the ball differently on the court. I think that he's doing that incredible."On the other hand, the bonus for him is that he's physically the strongest player on the tour, and mentally he has this incredible ability to stay focused from the first to the last point."But I don't think that he's unbeatable," added Djokovic, whose own confidence has been buoyed by a victory in Dubai last month. "He deserves to be number one in the world right now, but there are players that can win against him, especially on this surface."Djokovic proved that last year, but he got a first-hand reminder at just how strong Nadal is as Spain beat Serbia 4-1 in a Davis Cup tie in Benidorm last week.Nadal, who won't open his campaign here until Sunday, said he was settling in nicely and getting over jet lag after flying in on Tuesday. He was up before daybreak on Wednesday and Thursday - relaxing in the Jacuzzi of his bungalow and getting in a little golf at dawn - but by Friday the Spaniard said he had adjusted to California time. He has also adjusted to being the man to beat at any tournament he enters. "My feeling didn't change too much, being number two or number one," Nadal said. "Sure, if I can continue to be number one, it will be very nice. But my goal is to be a better player for all the tournaments, no?" Nadal said gaining the top ranking hadn't meant an end to his efforts to improve his game. "I always work on all aspects," said the 22-year-old, who will be vying to regain the title he won here in 2007. "Play more aggressive, serve a little bit better, be more complete player. "If you can slice, you have another option. If you can go to the net, it's another option to do. When you have more options, you have more resources. When you have tough matches that's really important."