But the Spaniard was impressive on Friday, finishing with 35 winners and 19 unforced errors to Del Potro's analysis of 20 and 32.
Nadal crushed Del Potro's spirit with the intensity of his tennis in the second set, bounding 5-0 ahead before the Argentine registered a game to lift the spirits of his fans.
His huge serve helped him to save set points too and he finally gained the upper hand to take a two-set lead.
Although at times during the first set against Del Potro, especially when he was 0-40 down at 1-1 and 15-40 down at 4-4 it looked as though he might be in for a real scrap. Injuries have hampered the King of Clay throughout the past year. In the fourth game Friday, del Potro clutched at his left hip after being wrong-footed by one shot from Nadal and was visited by a doctor at the next changeover.
Thiem has beaten Nadal on clay in each of the past two seasons, including just a few weeks ago on clay in Madrid, so that will certainly boost his confidence. Nadal is 49-0 against everyone else on his preferred surface in 2017 and 2018.
"I am very happy to be in another Roland Garros final, it is the most important tournament of my career".
"There are always things to learn, you don't lose energy", Thiem once explained in defence of his gruelling schedule.
And on the penultimate point of the semifinal, after being run ragged, the 2009 US Open victor could only offer up a smile.
It will also reignite the debate over who is actually the greatest player of all time.
He is the only man to beat the world number one on clay in the last two years - in Madrid this spring and in Rome last year.
However, it was Del Potro who was in the ascendancy in the rallies in the first set.More news: IOS 12, macOS Mojave, Memoji and Other Big Announcements from WWDC 2018
Rafael Nadal admits that his desire to clinch an 11th French Open title on Sunday is fired by his growing realisation that the end of his career may not be too far away.
If Thiem wins on Sunday, he will be the youngest man to win at Roland Garros since Nadal won at 24 in 2010. Thiem has won 7-5 6-3 and lost creditably 7-6 6-4 in Madrid, and won 6-4 6-3 in Rome.
The sixth-seeded Frenchmen have won two Grand Slams together.
This time, however, he lost it 12/10 despite having three set points.
It was the Austrian's third consecutive appearance in the semifinals at Roland Garros.
Nadal, a 16-time Grand Slam champion, claimed his 85th win in Paris against just two losses after weathering a big-hitting opening from 2009 US Open victor Del Potro to win 6-4, 6-1, 6-2.
More prize money is up for grabs than ever before at this year's French Open but it remains the least lucrative of the four Grand Slams for players to win.
A tired Del Potro looked like he could not take any more punishment when a Nadal backhand victor gave him a break to love in the first game of the third set. Their semifinal is later Friday.
"He's an awesome player", Nadal said of Thiem.
Thiem is bidding to become the first Austrian man to win a Grand Slam since Thomas Muster triumphed in Paris in 1995.
Victory would give Nadal an 11th title in Paris and equal Margaret Court's all-time record for wins at the same Slam event - in her case, the Australian Open from 1960 and 1973. We see each other every year in Vienna. Nadal -5.5 games is available on the Exchange at [1.68] and given the fact that Nadal has tended to dominate Thiem on non-quick clay, this should be a very likely outcome if Nadal wins the final.