Rafael Nadal is still far from his best. [Photo/Agencies]

Rafa Nadal's worst defeat on a claycourt for nine years offered ammunition to the naysayers but the Spaniard says his recovery from knee injury should be judged over time.

The seven-time French Open champion's narrow loss to Argentine world No 73 Horacio Zeballos in Vina del Mar on Sunday was rightly regarded as a shock as Nadal's defeats on clay over the past eight years can be counted on two hands.

Even those rare reversals have come against players of the caliber of Roger Federer and, most notably Novak Djokovic in 2011 and the last time he lost to a player outside the world's top 20 on clay was five years ago in Rome when former world No 1 Juan Carlos Ferrero got the upper hand.

The analysis began immediately afterwards with all eyes focussed on Nadal's movement for signs of the left knee problems that meant he had not played since a shock defeat at Wimbledon last year to Lukas Rosol.

Worryingly for Nadal's millions of fans, the 26-year-old openly admitted the knee was still causing him pain.

"The knee is still bothering me, but you have to face adversity with the best possible face and look forward to keep working and enjoy what I like the most, to play tennis," world No 5 Nadal said.

Whether or not the pain is just the result of not playing competitively for so long, or something more serious, remains to be seen but Nadal was clearly relieved he could compete at a high level in the nine matches he played in six days, including doubles.

"A week ago we didn't know how the body would respond," the 11-time Grand Slam champion said. "At least now I know we can compete at a certain level. I think that was a positive week.

"I will try to keep improving my physical sensations day by day, which is the most important thing because I don't feel that my tennis level is bad. I need more time on court."

Nadal's defeat was only his fifth in 41 clay-court finals with the others coming against Federer and Djokovic.

Nadal moves on to Sao Paulo this week for an indoor clay-court tournament and after a week off completes his south American comeback in Acapulco, Mexico.

Those two tournaments may offer a more reliable guide as to the chances of Nadal reaching peak from in time for the European clay-court swing in which he did not lose a match last year.