Del Potro then won six of the next seven points to tie the match and force a fifth set.
Dolgopolov, who has been interviewed by anti-corruption investigators about a match with unusual betting patterns last month at another tournament, was hesitant to give Nadal too much credit. "That's the first time I experienced that".
Del Potro said he was seriously considering retiring in the middle of the second set, not even an hour in.
"To win in straight sets against Feliciano, who's been such a tough competitor over the years, is such a great feeling".
"I'm sure there were many Argentinians in the crowd and some different reasons, for his success and he has come back from injuries and his game is attractive".
Federer believes that had Del Potro remained fit he could have become world number one after that breakthrough. A newfound energy guided him to another break of Thiem (pictured below) for 5-1 and all of a sudden the match was into a fourth set.
These two have played 21 times and Federer has won 16 of those meetings.
Even Roger Federer, playing his fourthround match simultaneously at Arthur Ashe Stadium, said he was intrigued by the del Potro-Thiem match. I am in the second week. "It was a great match I think, set four and set five. I can play really fast", the teenager warned his 31-year-old opponent. That's what I did in the end, just keep fighting.
"Of course I know that I have a good draw here", said Carreno Busta, who beat Canadian teenager Denis Shapovalov in three tiebreakers in the fourth round.
Dolgopolov, who beat Viktor Troicki 6-1 6-0 6-4, is one of the more surprising names in the fourth round and the Ukrainian's run comes despite the match-fixing investigation hanging over his head.
Federer had already won the French Open, to complete the career Grand Slam in 2009, before going onto to win a sixth Wimbledon. In the matches like this one, this might be the crucial thing. "I will try to do my best - I have nothing to lose", said Rublev, whose push to victory on Monday was helped by Goffin suffering a left knee injury. "I moved to Barcelona to practice with a new coach and new physio, a new fitness coach, all the new team".
16 seed Anastasija Sevastova of Latvia 6-3, 3-6, 7-6 (4) on Tuesday to reach the final four of a major tournament for the first time since 2013.
While Del Potro's form over the a year ago has been good, he bombed out of both the French Open and Wimbledon early, and is out to make amends at a tournament he won in 2009. It's a very special tournament to me.
"I saw the crowd waiting for more tennis, waiting for my good forehands, good serves", he said.
"[It's] going to be interesting match [to] play". But when he was hurt, clearly I didn't see him for a long time.
The former world number one might have struggled early on in the US Open, but his form during the season has been nothing short of magical.
Del Potro defeated Thiem previous year when the Austrian was forced to retire due to injury in the second set, but Thiem is carrying stellar form in NY after downing Frenchman Adrian Mannarino in straight sets on Day 6.