He said: "If I lose tomorrow, I am happy about my US Open. It wasn't an easy situation for me coming to this tournament after not having an easy summer.
"I am doing a lot of things much better than a few weeks ago. For me to win is important, but to feel myself very competitive and have the feeling that I can win is probably even more important.
"Because if you had that feeling that you can win, you don't win this week, maybe not next one, but at the end you will win."
Nadal has won 12 of his 16 meetings with Murray including the last four. It will be the third successive grand slam the pair have faced each other in the last four, with the man from Majorca triumphant at both the French Open and Wimbledon.
Murray feels hard courts present his best chance of defeating Nadal, and the world number two knows he will have a battle on his hands.
"If he's closer, he will beat me because on clay it was very close, and on grass it was a very close match," said Nadal, recalling their Wimbledon battle.
"He was beating me by one set, and I remember an important mistake with his forehand, it was an easy forehand to put him 15-40 at the beginning of the second set, so it was a really, really close match.
"He's a fantastic player on all the surfaces and it will be a very tough match for me and hopefully for him, too."
Both men will be playing their third match in as many days, but Nadal insisted that did not enter into his thinking as he thrashed Roddick with a relentlessly aggressive display.
Nadal went into the match without having dropped a set but there were still questions about his form given none of his previous opponents had been seeded.
Roddick had beaten fifth seed David Ferrer in the fourth round and had looked in good form but he was simply blown away by the power and accuracy of his opponent.
It was an impressive performance from Nadal, perhaps thinking about saving energy with a third match in three days to follow tomorrow.
The Spaniard was on the front foot from the off, breaking Roddick in the opening game and reeling off four in a row before the American, who was being passed time and again at the net, finally got on the board.
The pattern repeated itself in the second set, with Nadal getting off to a flying start, and this time he broke the big-serving American three times.
The second seed's charge was held up briefly when Roddick took a medical time-out to have his thighs massaged, but by that point Nadal was 3-0 up in the third set and cruising.
First lady Michelle Obama, watching from the stands, was no doubt hoping for a home win but a comeback never looked on the cards and Nadal wrapped up the most convincing of victories with his 35th winner.
Nadal said: "I don't think about having a short match in the quarter-finals of a grand slam. I think about trying to win the match. I think I started the match really well with some very good points, a few very good passing shots.
"It is always a very hard match against him, and probably he was tired. That happens when the rain comes. I think I did well. I played a very good match I think."
Roddick came into the tournament with little tennis under his belt because of a stomach muscle injury and he ran out of steam, but he was proud of his efforts this fortnight.
The 2003 champion added: "From six weeks ago to a quarter-final and a top-five win, I don't know if I thought I would do that.
"Thankfully last week I was able to play matches to at least where I was feeling the ball. I think it was always going to be tough for me to play that many three-out-of-five-set matches on no training, pretty much just sitting around.
"But I felt like I did what I could from the injury. I feel like I gave myself every opportunity, and it was enough for a good result."