The Spaniard will play either defending champion Novak Djokovic or Italian Lorenzo Sonego in what will also be his 52nd Masters final.
Playing his 500th clay-court match, Nadal staved off break points in the fourth game of the first set and did not look back to win through in 1hr 32min.
Earlier, world number one Djokovic survived a three-set battle with Greece’s Stefanos Tsitsipas over two days in their rain-delayed quarter-finals to advance to the semi-finals.
The defending champion won through 4-6, 7-5, 7-5 after 3hr 16min on court in the clash which had been suspended from Friday because of rain in the Italian capital.
The Serb, a five-time Rome winner, had been trailing 6-4, 2-1 when the game was halted overnight. Djokovic twice recovered from a break down in the final set to seal a berth in the final four for the eighth straight year on his second match point.
The 33-year-old next meets Italian Lorenzo Sonego who shocked Russian seventh seed Andrey Rublev 3-6, 6-4, 6-3 in 2hr 33min.
“It felt like we played two matches, actually we did, one yesterday where he was a better player,” said Djokovic.
“He started also better today I managed somehow to hold my nerve, breaking serve at important moments.
“I am just really, really glad to overcome this challenge. It was probably the toughest match of the year for me so far.”
Monte Carlo winner Tsitsipas had moved to a 4-2 lead in the second set on their return to court Saturday, but Djokovic made a crucial break for 4-4, the Serb saving break points in the following game for 5-4 and breaking in the 12th game for 7-5 to level at one set all.
Both hit 15 winners in a balanced second set but Tsitsipas paid for four more unforced errors than his rival who used his forehand to break through with winners along the line.
Tsitsipas twice led by a break in the deciding set and served for the match at 5-4, but winner Djokovic rallied to take a 6-5 lead.T sitsipas served to force a final-set tie-break, but the 18-time Grand Slam winner again proved too strong for the Greek who has won none of their three meetings on clay.