The 33-year-old produced a masterful performance to get past Federer 6-3, 6-4, 6-2 in the last four on Court Philippe Chatrier and set up a final clash with either world number one Novak Djokovic or Austrian fourth seed Dominic Thiem on Sunday.
The pair will instead restart their match at 1000 GMT (12:00 local time) on Saturday, before the women's final between Czech teenager Marketa Vondrousova and Australian eighth seed Ashleigh Barty. "That's all I can tell you", Djokovic said.
Thiem's coach Nicolas Massu said he would prefer the final to be shifted to Monday but the Austrian insisted he was happy to play Sunday.
The Austrian is widely recognized as being an improved player from past year, and heads into the encounter ranked fourth in the world.
Thiem completed a 6-2 3-6 7-5 5-7 7-5 triumph, in a match that began on Friday but spilled into the weekend due to rain in Paris.
Thiem said he would "leave everything he has out on court" to try and stop 11-times French Open champion Nadal, but added a dose of realism as he looked ahead to probably the toughest task there has even been in tennis.
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He proved his grit and determination against Djokovic - himself arguably the most dogged competitor out there - by prevailing in four-hour semifinal, despite a wobble in the fifth set that could have changed the outcome.
"[Friday] was one of the worst conditions I have ever been part of".
Austria's Dominic Thiem had reason to celebrate Saturday after weathering yet another rain delay to defeat world No. 1 Novak Djokovic on Day 2 of their rain-halted semifinal at Roland Garros. While Nadal is seeking his 18th Grand Slam title, the Austrian fourth seed Dominic Thiem is seeking his first, hoping to better his best finish at a Grand Slam when he had reached the final past year on the Parisian clay. On that occasion the Spaniard edged a tight first set to win through in straight sets for an 11th Roland Garros title and a 17th major overall.
In gusty winds Thiem's powerful groundstrokes helped him dominate the first set and Djokovic was in trouble early in the second when he fended off a break point. "And they said there is no chance", said the Chilean. "I'm really looking forward to tomorrow, to let everything out on the court again".
On Friday night, he had called the supervisor onto the court to complain about having to play in the high winds.
"I've been telling everyone - if someone wants to beat Thiem they have got to be ready for a big battle".