The Spaniard made an instant impact on the encounter when he broke Nieminen's serve at the first time of asking to put him 2-0 in front.
Nieminen responded by holding his remaining service games, but he was unable to regain the game that he lost as world number two Nadal served out for a 6-4 success.
Just as he did in the opening set, Nadal broke his Finnish opponent early in the next.
The 25-year-old, who has won this tournament seven years on the trot, then claimed another of Nieminen's service games to open up a 5-2 advantage and leave him serving for the match.
However, a high lob from Nieminen in the next game caused Nadal to strike his return into the net and allow the Finn a chance to break serve, which he subsequently took.
He failed to build upon that success, however, as Nadal broke straight back to win the match 6-4 6-3.
Nadal will now face Kazakhstan's Mikhail Kukushkin in round three.