In his 2003 debut on the dirt, Federer was close to faultless, not dropping a set at an event where he was never really tested. The 21-year-old also claimed titles indoors at Marseille and on hard courts in Dubai in February.
"I'm glad to have won, Nieminen's a dangerous player on clay," said Federer, fifth in the ATP Champions Race heading into next week's Rome Masters. "It was no surprise to find Jarkko in the final. I had to work hard, he hits the ball so well.It's great to already have three titles this season."
Nieminen has now come up short in four finals - the last three of them played on clay - including two contested in April, 2002 in Estoril and Mallorca.
"It shows my game is there if I reached another final, but I'd like to win one," Nieminen said.
Federer has lost only six matches this season while posting 31 victories. It was his third career win over Nieminen after Moscow last autumn and in the Marseille second round two months ago.
The Swiss did not allow Nieminen to hold serve once in the 24-minute opening set, setting an untouchable pace with his big game.
Federer fought back from 0-3 in the second set, quickly putting things right on a sunny afternoon. He broke for 5-4 and served out the win, recovering from 0-30 down.