The 27-year-old Spaniard clinched his second US Open, and his 13th career major, with a 6-2, 3-6, 6-4, 6-1 win over Novak Djokovic. (Agencies)
Nadal completed a career Grand Slam in New York in 2010, but winning all four majors in the same year has only been achieved by Don Budge (1938) and Rod Laver (1962 and 1969).
"To win all four Grand Slams in one year I think today is impossible for anyone. That's my feeling," said Nadal, who now has eight French Opens, two Wimbledon, two US Open and an Australian title amongst his 60 career crowns.
"Today the best players are there all the time, so to win a tournament like this you have to win against Roger (Federer), against David (Ferrer), against Andy (Murray), against Novak.
"These players are not losing in the early rounds, so that makes it impossible be 100 percent in every tournament. So when your level is a little bit lower, you will lose against these players 100 percent," he said.
Sunday's win capped a memorable year for Nadal, whose season only started in February after a seven-month injury layoff, an absence which saw him miss the 2012 US Open as well as the London Olympics and the Australian Open this year.
But he now has 10 titles since his comeback, a 60-3 match record and has won all 22 matches he has played on hard courts. That run saw him also win a record eighth French Open and five ATP Masters titles.
The 27-year-old Spaniard clinched his second US Open, and his 13th career major, with a 6-2, 3-6, 6-4, 6-1 win over Novak Djokovic.