But Nadal, second only to Federer in career major-tournament titles with 15, quickly forced a tipping point and powered to a 4-6, 6-0, 6-3, 6-2 victory Friday night. It's been a very emotional year.
"It's been an wonderful season for me and I'm so happy to get a chance to fight for another title after a couple of seasons with injury troubles", Nadal said after his match.
"This means the absolute world to me", he added. "I was not playing bad in the first set but I lost it so some things had to change".
"So I tried to move him around more and be more unpredictable".
Big-serving South African Kevin Anderson was spotted on a practice court stretching his 6-feet 8-inches (2.03 metres) frame as he prepares to meet the other Spaniard in the semi-finals, Pablo Carreno Busta.
Nadal has dropped just two sets in the tournament so far and was never troubled by his Russian opponent, who hit 43 unforced errors during the contest.
"He had some injuries so he's a big example for the kids and for the rest of the tour, too". Anderson confidently held his serve and reception made four breaks and got out to the main end of his career.
He will now have the opportunity to secure a third US Open title and 16 Grand Slam crown on Sunday (September 10).
Friday's loss was his ninth in 14 meetings against Nadal and he was quick to tip his conqueror to defeat Kevin Anderson in Sunday's final and add to his 2010 and 2013 titles in NY.
Nadal, the US Open champion in 2010 and 2013, said: "For me, what is more important, more than winning slams, is to be happy".
"Then to break his serve in the fourth, and I hit my best backhand on the tournament tonight, which is a good signal for the future". It happened in 2009, when del Potro himself beat Nadal in the semifinals, and then Federer in the final. He needed five sets to win each of his first two matches - and Wednesday, he succumbed to the del Potro's familiar formula of massive forehands and booming serves. "Maybe at the beginning he did a lot of mistakes, but I played aggressive, too. The player who plays better that day is the player that is going to have better chances", mentioned Nadal with a philosophical approach ahead of the match. The Argentine tennis star, often referred to as a giant killer, took just four sets to defeat the legendary Federer in the last round.
Nadal, meanwhile, understands what a threat Del Potro poses, as the Argentinian showed when he beat Dominic Thiem and Roger Federer in his last two matches despite suffering from flu.
"Tomorrow is going to be a different match than the past", he said. Juan Martin del Potro prevented it once and can do it again.