Melbourne, Jan 23(Agencies): World number one Rafael Nadal said he will have to improve if he is to win his fourth straight Grand Slam title as doubts persist about the state of his health at the Australian Open.

Australian prodigy Bernard Tomic made the Spanish top seed sweat as he took two-and-a-half hours in an intense three-setter to get through to the fourth round late Saturday.

Nadal worked himself hard as he struggled at times against the 18-year-old Tomic, ranked 199, but with a buoyant future beckoning.

"I didn't play well. I played short. My movement wasn't the best. The position on court wasn't the best neither," Nadal said.

The nine-time Grand Slam champion said he was satisfied with his fighting spirit, especially when Tomic got a double break to lead 4-0 early in the second set before Nadal reeled him in.

"The positive thing was that the attitude was good all the time," he said.

"Even down 4-0, I kept fighting for the second set and at times I did okay in the third. I went to the net more than before, so that's a positive thing.

"But I have to change a few things if I really want to have chances to win the next match, and I'm gonna do it.

"Every day it's going to be really difficult. The next match is going to be very difficult and I have to be ready to play my best tennis."

Nadal on Monday takes on Croatia's 15th seed Marin Cilic, who outlasted American John Isner in five sets over four-and-a-half hours in the third round.

Cilic beat Nadal in their only meeting in straight sets in the semifinals of the 2009 China Open in Beijing.

Nadal is bidding for a fourth straight major title to complete a non-calendar year Grand Slam.

Nadal, who saturated two shirts while playing Tomic, revealed that he was still not over a virus-type illness which he developed on the way to Australia.

"In Doha I had that problem. I wasn't feeling very well. I had a fever," he said. "It seems like after that my body is still not perfect. I am sweating more than usual. I am more tired than usual when I'm playing."

Nadal said he was unsure why he had not recovered more quickly.

"I don't know. Hopefully I'm going to improve. I don't know what's going on," he said. "I am doing all that I can. I am working, I am resting. I can't do more.

"If I don't feel perfect, it's not my fault. All I can say is keep fighting and maybe I will be better next week."