The New York-born Daniel booked a spot in the second round of the season's last Grand Slam on Wednesday.
The New York-born Daniel booked a spot in the second round of the season's last Grand Slam on Wednesday when he battled past American wildcard Tommy Paul 6-1, 4-6, 4-6, 6-2, 6-2.
However, his celebrations didn't last long.
"When I got off court, I didn't know who I would be playing. But then I found out it was Nadal and I thought, 'Oh God!' I didn't realise it," said the 24-year-old.
"It's really exciting but I need a few hours to let it sink in."
"I know it will be on a big court so that will be exciting. But it could also be nerve-wracking."
"I need to go in excited and just hope that he doesn't come out too powerful."
Daniel, who says in his biographical notes that if he wasn't a tennis player he would have liked to have been the guitarist with Led Zeppelin, hopes to thrive on the rock concert atmosphere that is a trademark of the US Open.
"Most of my American fans are out in California. But I was born here so there should be something about the place," said Daniel, whose parents met on Wall Street before relocating to Japan in his childhood.
The globe-trotting Daniel, ranked at 121, now lives and trains in Valencia and speaks fluent Spanish.
His win over Paul was his first at the US Open after his only other appearance at the tourmanent ended in a first-round exit in 2014.
He came through qualifying that year but was stopped by Canada's Milos Raonic in his opening match.
Wednesday's win was also his first since he made the second round at Roland Garros in May after also getting out of qualifying.
Daniel was joined in the second round by compatriot Yuichi Sugita, who also won his first ever main draw match at the US Open by seeing off French wildcard Geoffrey Blancaneaux, 6-2, 6-2, 6-0.
Sugita had tried and failed to qualify in New York for eight straight years from 2009-2016.
With a world ranking of 44, the 28-year-old late bloomer is his country's top men's player at the tournament after 2014 runner-up Kei Nishikori withdrew with injury.
Sugita will face Argentine lucky loser Leonardo Mayer, who downed French 26th seed Richard Gasquet 3-6, 6-2, 6-4, 6-2 for a place in the last 32.