Rafael Nadal said it will be "very tough" to seize Novak Djokovic's crown as the top ranked men's tennis player, paying tribute to his "unbelievable" opponent at the China Open on Sunday.
The world number two can unseat Djokovic from the top spot with a good performance in Beijing, but the Serb will go into the tournament with confidence he can retain the title he won last year.
Djokovic is unbeaten in three visits to the China Open, following up wins in 2009 and 2010 with victory over Jo-Wilfried Tsonga in last year's final.
Spaniard Nadal has been in fine form since coming back from injury earlier this year, and defeated Djokovic at this month's US Open final.
But the 2004 champion in Beijing says overcoming Djokovic is no easy feat.
"I know that my opponent is unbelievable. He's able to not lose many matches during the whole season and it will be very tough to become number one," Nadal said to reporters in Beijing.
"But if I am still playing the same way that I did the last couple of months, I hope to have my chance here or in the next few weeks."
The men's tournament at the dual WTA and ATP was still at the qualifying stage on Sunday.
Djokovic is the top seed in Beijing, ahead of number two Nadal.
The reigning champion will play Czech Lukas Rosol, who is ranked 46th in the world, in his opening game, while Nadal will face a qualifier.
The women's tournament at the China Open began on Saturday. The tournament reach its climax on October 6.