Thiem, who has never made it to the final at another Grand Slam, is looking to become the first Austrian to win a major since Thomas Muster lifted the Musketeers Cup at the French Open in 1995.
Men's semifinal: No. 4 Dominic Thiem beat No. 1 Novak Djokovic 6-2, 3-6, 7-5, 5-7, 7-5. He remains flawless in the final at the French Open, and this is the second straight year in which Nadal has beaten Thiem in the championship.
The second semi-final was suspended until Saturday due to rain, with Thiem leading 6-2, 3-6, 6-1 as he looks to reach a second straight Roland Garros final.
Thiem did manage to get one game on the board but was powerless to avoid another comprehensive set win for the Spaniard.
Barty didn't fold and had more in the tank to recover to win, 6-7 (4), 6-3, 6-3.
Nadal's response was brutal as he won 16 of the first 17 points of the third set against a tiring Thiem, bagged it in less than half an hour.
More than an hour after the women's final should have started on another stormy day on Court Philippe Chatrier, fourth seed Thiem thumped a forehand victor past Djokovic to convert his third match point.
Austrian fourth seed Thiem, his victim in last year's final, has enjoyed none of those advantages, needing two weather-disrupted days and five sets to get rid of world number one Novak Djokovic in the other semi-final.
A tally of just nine games meant it was Federer's heaviest defeat in a Grand Slam match since managing only four against Nadal in their famously one-sided 2008 Roland Garros final.
He got himself back together to hold for 6-5, and then forced a third match point against Djokovic, which he sealed by drilling a final forehand victor.
The 15-times Grand Slam champion regained his composure to break serve twice in the fourth set but on both occasions the attacking Thiem hit back immediately.
The Austrian did dig deep though and he had the opportunity to break Nadal in the first game of the fourth but the 11-time victor responded in style, producing a vital hold.
Thiem won his last match on clay against second-seed Nadal, but that was the best of three sets.