His audio interview:
http://www.fft.fr/usopen/2008/?id=4017
He sounds really happy about his performance.

I played a very good match, I was feeling my shots very well, I felt I could do really great things today, I was very solid when I needed to, etc. etc. etc.
Since he was feeling so well, he tried out other things, like coming more to the net and he sometimes attempted to do too many things at once, hence the little hiccups in the scoreline. But nothing worrying.
Chucho is a very good player from the baseline, but he knows how to play so Chucho can't hit a lot of winners against him and gets tired. He was looking quite tired again today after 2 hours of play.
Then he's asked about Del Potro and answers Del Potro hasn't won yet. But he isn't afraid anyway. He'll have to be careful, but "he's a tennis player, playing with a tennis racquet, like me". "He's a very good player, but I think I am a very good player too.". (That's vintage Poussin today!

)
He already played him last year and it was a very long match, 3h30. He found Del Potro very good from the baseline last year, very consistent and it was a great fight, but he was a bit nervous on his service games. And he completely fell apart physically at the end of the 3rd set.
He'll try to "involve him in a physical fight". But he can't afford any mistake this time. Del Potro has obviously improved his serve a lot. He'll need to be strong on his service games and make the match last as long as possible.