Despite claiming his seventh straight Monte Carlo Masters title, world number one Rafael Nadal said he must keep at improving his game.

The Spaniard beat fellow countrymen David Ferrer 6-4 7-5 in a hard-fought encounter on Sunday but he looked far from his best on clay, which is his favorite surface.

I'm pleased with how the first week (on clay) has gone. He said. I need to use the confidence from the first week to keep improving. I have to be more aggressive, to serve better and to finish off points more emphatically and with more authority.

Nadal could face the in-from Novak Djokovic and old rival Roger Federer on three more occasions on clay - in Madrid, Rome and the Roland Garros in Paris. However, it is Djokovic more than Federer, who is seen as a bigger threat to Nadal's number one status. The Serb has a perfect record so far in 2011, winning 24 successive matches including victories over Nadal at Indian Wells and Miami finals.

With the clay court sure to test the endurance of Djokovic's form, their clash is eagerly awaited. When Nadal was queried about how Djokovic rather than Federer was seen as his main rival these days, he said, He (Djokovic) has started very well and I congratulate him. Roger and I have played many important matches, with great moments for each of us. The year is very long and he (Djokovic) is in a privileged position but unfortunately I have to worry about many others as well.

I'm not invincible on clay. I have lost before and unfortunately I have plenty more games I will lose in the future. It has gone well for me in recent years but no one is perfect.