With Rafael Nadal and Carreño Busta reaching the semifinals, it marks the first time in US Open history that two Spanish men have reached this stage.
The 31-year-old, ranked 32 in the world, is the lowest-ranked grand slam finalist since 38th-ranked Jo-Wilfried Tsonga fell at the 2008 Australian Open final.
He might benefit from picking Nadal's brains before his semi-final match against powerful South African Kevin Anderson tomorrow morning.
"I've only been on the job since the end of previous year and since then we've really embarked on a building phase within Tennis South Africa".
"I think to a certain extent that is a valid criticism, and one that we have to hold up our hand and acknowledge". It was tough for Pablo and me. "I know I didn't win matches against top players, top-10 or top-20 players, but I am very happy with my tournament".
"I'm over the moon".
The Spaniard maintained the pressure, a forehand victor down the line capturing an early break and a 2-0 lead in third set.
It looked as though the Spaniard might prevail when he responded to finally losing serve midway through the second set by hitting straight back, thanks to a forehand victor arrowed down the line.
It was now the South African making all the running, earning six break points in the third set.
This will be the second time in less than a month that Anderson will meet Carreno Busta. She went on to win it by 6-1 without facing a single break point.
Anderson improved to 3-0 against Carreno Busta, but he hasn't had any success against either Nadal or del Potro.
They also met in the 2009 semifinal at Flushing Meadows, when del Potro routed Nadal before beating Roger Federer for the title. Anderson has been claimed as one of the game's best servers now, and he leads the men's field in the most number of aces served.
The No. 24 seed won the first set 6-4, winning 17 of 22 points on his first serve and putting it away with one of his powerful forehand winners. He attacked with blood in his eye and he defended as if his life depended upon it. Del Potro, increasingly tired as the sets flew by him, could do nothing to stop Nadal's march to the final.
"When Juan Martin is playing well, it's hard to stop him".
For years Anderson had to endure unwarranted criticism that he has chosen self-interest over that of his country. I have given myself a shot at being in the finals and I will be playing for a grand slam trophy.