By Simon Evans
"I took a lot of risk there in the last two sets. That's what I had to do," said Roddick, who has reached his second consecutive Masters series final after losing to Croat Ivan Ljubicic at Indian Wells last month.
"My comfort zone of moving the ball around and maybe chipping it around a little bit doesn't work against Rafa. I had to try to come up with something that at least took him out of his comfort zone a little bit and it paid off," he added.
Nadal has not won since the Rome Masters in May but the fourth seed looked poised to take a step toward the final after dominating the opening set in gorgeous sunshine.
Nadal had won five of their previous seven meetings in the last six years and that looked a good guide to the outcome of this encounter after he broke Roddick's serve in the third game -- a magnificent crosscourt forehand securing the break.
The Spaniard went on to win the set but it was a different story in the second where Roddick broke in the eighth game with a pair of outstanding forehands taking him to another level.
Brimming with confidence, the 27-year-old from Nebraska took the game to Nadal in the third and that positive approach was rewarded when he broke to go 2-1 up after the under-pressure Spaniard dropped a straightforward forehand return into the net.
Roddick kept his serve strong and then wrapped up the victory by breaking the struggling Nadal again.
The Spaniard, beaten by Ljubicic in the semis at Indian Wells, was pleased with his displays despite going out.
"I just have to keep working like this, keep improving. You know, two semi-finals in a row, first two Masters 1000's of the season for me, is positive and it has been a positive American hardcourt season for me.
"I am playing well. I had very good matches in these two tournaments -- those are the positive things. The worst thing is I lost two semi-finals in three sets. That's it.
"But if you are there, you are going to have your day. You're going to win one day," he added.
(Editing by Peter Rutherford and Ken Ferris)