"I played a really normal match, nothing special. Nothing very bad, but nothing very good," said Nadal, who improved his overall record here to 40-1, including 38 straight wins. "First match on clay after a long time without touching a racket."
Nadal's last match was against Jo-Wilfried Tsonga in the Sony Ericsson Open quarterfinals. He pulled out of his semifinal against Andy Murray to get treatment for a knee tendon problem, having skipped a chunk of the season to rest his knees following the Australian Open.
Nadal broke Nieminen's serve four times, and the Finn had a double-fault on match point.
The Spaniard next plays Kazakhstan's Mikhail Kukushkin.
"I need practice, and today was a good practice," Nadal said. "Tomorrow (is) going to be another tough match against a player who is dangerous."
Since 2005, Nadal has won at Monte Carlo and Roland Garros every year except 2009.
Djokovic took a 4-0 lead after 19 minutes before Seppi rallied in the second set. But Djokovic closed out the match on serve to improve to 7-0 against Seppi.
"I haven't played a match on a clay court since Roland Garros last year, so I'm happy with how I played," Djokovic said.
Djokovic says to "expect the unexpected" with Dolgopolov.
"He's a very talented player, comes up with some shots that are not really typical for a tennis player," Djokovic said. "He comes up with some drop shots, changes the pace very much. That's why he's very dangerous on clay."
The 23-year-old Dolgopolov is confident of causing an upset. Although he lost in straight sets to Djokovic in the fourth round of last year's U.S. Open, the Serb had to work hard to take a tight first set 7-6 (14).
"After that, I think I can compete with him," Dolgopolov said. "I mean, he's No. 1, I'm No. 21. Obviously, he's higher in the ranking. He's playing more consistent. But I think I have my chances."