Spain's Rafael Nadal was down a set and a break against the Argentinian Diego Schwartzman in the quarter-finals.
And if Federer had 20 Grand Slam titles, Nadal is breathing down his neck again with 17.
That's it. There is the history of the King of Clay's losses at the French Open.
Siniakova was the doubles runner-up at last year's U.S. Open with a different partner.
It was hard not to feel sympathy for Thiem, playing in his first Grand Slam final against a man who simply does not lose this particular battle.
Nadal won the first two sets and was leading 2-1 in 3rd when he suffered a cramp to his left wrist which required immediate medical timeout.
"Let me enjoy this title", Nadal said. I didn't see much from the finals because I arrived here, warmed-up and played by myself.
It rarely is for Nadal at a place he has lorded over the way no other man ever has at any Grand Slam tournament.
"So coming back and have the chance to win in Monte Carlo, Barcelona, Rome, and now especially here, yeah, it's very emotional for me".
Nadal avenged his defeat to the Swede at Roland Garros 12 months earlier.
That shock exit prompted speculation that Nadal was finished as a heavyweight of the sport.
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He also regained the world number one ranking for the first time since July 2009.
The Austrian is one of only three players to down Nadal on clay at least three times; the world No. 8 was the last man to overcome Nadal on clay, last month in Madrid.
Halep's coach, Darren Cahill, has said the Romanian enlisted the help of Murray's psychologist, Alexis Castorri, to help land her first major title, per Simon Briggs at the Telegraph.
It was also a rematch of last year's U.S. Open final, in which Stephens topped Keys.
In the meantime, Nadal is scheduled to compete at Queen's Club next week, before bidding for a third Wimbledon title. It was his 45th career clay-court title.
- Victory on Sunday gave Nadal his 11th crown at Roland Garros from 11 finals. He has won every final he has reached, and in those 11 showpieces he has dropped just six sets.
Rafael Nadal described his performance at the French Open past year, when he didn't surrender a set, as "perfect".
Nadal is now level with Margaret Court for the record of the most titles at a single grand slam, while his total of 17 overall brings him closer to Roger Federer's 23.
Nadal struck first in an enthralling opening set, breaking Thiem's first service game to take a two-game lead but they were back on serve immediately as the Austrian hit back. "Incredible", Federer said of Nadal's victory.
However, he said: "I've had long and mentally tough clay-court season because I played nearly all the matches possible after coming from an injury". But at the same time, in the other hand, it's true that I went through tough moments, a lot of times in terms of injuries.