Nadal improved to 20-10 in his career against Federer, and showed once again that he'll be the player to beat when the tournament at Roland Garros starts next Sunday. It was the fifth-ranked Spaniard's fifth title since returning earlier this year from a seven-month layoff due to a left knee injury.
In the women's final, Serena Williams won her fourth consecutive title of the year in impressive fashion, defeating third-seeded Victoria Azarenka 6-1, 6-3. The top-ranked American will go to Paris on a career-best 24-match winning run.
Williams was coming off consecutive titles in Miami; Charleston, South Carolina; and Madrid last week.
She didn't drop a set while winning this title.
Federer had also not dropped a set all week, but he had no reply for Nadal's topspin-heavy groundstrokes. The 17-time Grand Slam winner attempted serving and volleying, but he either missed the volley or Nadal passed him with the return.
Federer lost more points, 10, than he won, nine, at the net. He also committed 32 unforced errors to Nadal's eight.
Center court at the Foro Italico was packed to the limit with 10,500 fans, but the crowd didn't get to see too much tennis. The men's final took only 1 hour, 9 minutes, and the women's final lasted only slightly longer.
On a pleasant spring day, Williams immediately took control by breaking Azarenka's serve twice to take a 3-0 lead in the opening set.
The 15-time Grand Slam winner slugged winners at will off Azarenka's first and second serves, stepping into the court to dictate play at every opportunity.
Azarenka grew distraught at the end of the first set, twice slamming her racket on the court in desperation.
After trading breaks midway through the second set, Williams took control again when Azarenka double-faulted to give her a 5-3 lead. Williams served out the match at love, letting out a big scream when she unleashed a backhand winner down the line to close it out.
"She definitely showed incredible tennis today," Azarenka said. "But I don't think the score says how close the match was. She was better at the key moments."
Williams held a 41-12 edge in winners and served nine aces to Azarenka's none.
Williams had twice won 21 matches in a row before, although they came more than a decade ago, in 2002 and the beginning of 2003.
Martina Navratilova established the longest women's winning run in the Open Era at 74 matches in 1984.
Williams' only previous title at this clay-court event came when she beat Justine Henin in the 2002 final.
That was also the year she won her only Roland Garros title. Last year in Paris, Williams lost in the opening round of a major for the first time, falling to 111th-ranked Virginie Razzano of France.
At 31, Williams is back at the top of her game after missing 11 months in 2010 and 2011 with a right foot injury and a pulmonary embolism.
It was Azarenka's first final since beating Williams for the title in February at Doha, Qatar. That followed the Belarusian's Australian Open victory. Since then, Azarenka has been slowed by right ankle and left foot injuries.
It was the 51st title of Williams' career. Navratilova also holds the record in that category with a seemingly insurmountable 167 titles. Williams moved within two titles of matching Monica Seles for ninth on the all-time list.
Wiilliams was already first among active players and now has seven more titles than her sister Venus, who is second on the list and watched the final from the stands.
Williams improved to 12-2 in her career against Azarenka, who spent 51 weeks at No. 1 before Williams reclaimed the top spot in February.
In the women's doubles final, Su-Wei Hsieh of Taiwan and Peng Shuai of China upset the top-ranked Italian pair of Sara Errani and Roberta Vinci 4-6, 6-3, 10-8. In the men's final, top-ranked Bob and Mike Bryan of the United States beat the sixth-ranked Indian pair of Mahesh Bhupathi and Rohan Bopanna 6-2, 6-3.