Spanish player Rafael Nadal warms up before a training session for the Monte Carlo Tennis Masters tournament in Monaco on Sunday. Lionel Cironneau / Associated Press

Clay king Rafael Nadal has admitted that he is nervous about his Wednesday start at the Monte Carlo Masters, a fortnight after quitting with knee pain prior to his last match.

"I'm scared because this is the start to an important season for me," said the Spaniard, who is bidding for an unprecedented eighth straight title on the clay of the principality. "Hopefully it will work well."

Nadal, seeded second behind Novak Djokovic, will play Jarkko Nieminen in the second round after the Finn beat Radek Stepanek 6-3, 7-6 (7-3).

In first-round play on Monday, all three seeds on court advanced.

Spain's No 13 seed Fernando Verdasco, a 2010 finalist against Nadal, beat Belgian Olivier Rochus 4-6, 6-2, 7-5, while 15th-ranked Austrian Jurgen Melzer defeated Lukas Kubot 6-2, 7-5.

Alexandr Dolgopolov, the No 16 seed, beat Argentine Juan Ignacio Chela 6-2, 6-2.

Frenchman Paul-Henri Mathieu, ranked 352 in the world after a long absence with a knee injury, crushed Donald Young 6-0, 6-1 to leave the American winless in his last five events.

After quitting before his Miami semifinal against Andy Murray on March 30 due to pain in his left knee, Nadal returned to Spain for injections and treatment of the tendon problem that was causing him grief.

He was unable to train for a fortnight and only got back onto the clay in the middle of last week.

"Now it's time to see how good it is," the 10-time Grand Slam-winner said.

"I need to be able to play at my top level and run without thinking about the knee. I've put all the effort in and hopefully it is well and I can train in the right conditions. That's the most important thing for me today."

The king of clay, whose last trophy came at the 2011 French Open, played down the fact that his last success came 10 months ago.

"I haven't won a title, but how many finals have I played?" asked Nadal, who has appeared in four finals since his Roland Garros triumph.

"It all depends on your calendar. If I played more on clay, like in South America or after Wimbledon, I would have better chances of winning titles.

"There are players there (at lower-profile events) who can beat you, but the chances are better. But I don't make my calendar to win titles, I want to compete at the most important events against the best players of the world.

"I'm not playing only on my best surface, but to try and win titles. That makes it more difficult. But I don't feel pressure just because I haven't won since Roland Garros."