* Nadal withdraws from ATP Masters 1000 event
* Aims to build up power in left leg (Updates with Nadal quotes, detail)
INDIAN WELLS, California, March 17 (Reuters) - Rafa Nadal has withdrawn from next week's Sony Open in Miami to strengthen his left leg over the next few weeks before beginning his build-up for the French Open.
The Spanish left-hander, who returned to the ATP circuit only last month after seven months out with a left knee injury, has played four tournaments in the last five weeks, winning three of them.
"The doctors recommend me to be back home for a few weeks, rest a little bit and keep practising the right way," the world number five told reporters after beating Juan Martin Del Potro 4-6 6-3 6-4 in the BNP Paribas Open final at Indian Wells on Sunday.
"You know, I need more power in the left leg quadriceps, so I need to keep working hard. The competition is hard for the body, so after four fantastic weeks I can't go to Miami. I need to prepare and rest for the next one."
Nadal said he would next be in action in Monte Carlo, followed by Barcelona, Madrid and Rome.
"That's my schedule, the four obligatories (including Indian Wells) and Barcelona," the 11-times grand slam singles champion said. "That's my schedule. I go week-by-week since I came back.
"For example, I cannot go to Miami. I'm very sad for that. I'm very sorry for the organization of the tournament and I'm very sorry for the fans, but we never thought we'd be able to play all matches possible since I came back."
Earlier in the day, Sony Open tournament director Adam Barrett had announced Nadal's withdrawal from Miami in a statement.
"We are disappointed to hear the news that Rafael Nadal will not be competing in Miami this year, especially given his strong results since his return to the Tour," Barrett said.
"He is a tremendous ambassador for the game and has thousands of adoring fans here in South Florida. We wish him well and hope to see him back in Miami next year."
Nadal's withdrawal from the second ATP Masters 1000 event of the year, like Indian Wells played on a hardcourt surface, had always looked likely due to his packed tournament run over the past five weeks.
He won two ATP titles on the clay of South America last month and, following his victory at Indian Wells, has improved his record this year to a career-best 17-1.
The claycourt specialist has won seven of the last eight French Opens and his focus will now be fixed on the year's second grand slam, starting in late May, as the ATP circuit switches back to clay after Miami.
"Clay, I mean this is the surface that I love to play and I had a great success in the past," the 26-year-old said. "But every year is a different year. Every year I have to improve.
"Every year I have to do different things if I want to keep winning on clay. And I try to do my best and I try to do different things and be better player every year to have the chance to make good results, no?
"So I'm going to go to Monte Carlo with the same expectations than ever, than always, and I'm gonna try my best to arrive there healthy and in good shape playing well." (Reporting by Mark Lamport-Stokes; Editing by Gene Cherry/Peter Rutherford)