World number Rafael Nadal and third seed Roger Federer are set to clash in the semis of the Sony Ericsson Open, which is incidentally their first clash on American soil in six years.

Federer advanced into the semis fairly easily after Gilles Simon withdrew with a stiff neck just 10 minutes into the game. Nadal had a much tougher time against Czech's Tomas Berdych.

The world number one progressed after a 6-2 3-6 6-3 win, in two hours and 17 minutes, fighting a strain in his shoulder, to set up the Federer clash.

Simon's retirement was greeted with boos from the Miami crowd, but Federer sympathized for the Frenchman.

He told me he woke up this morning with a stiff neck and couldn't move, said Federer. It's obviously disappointing when it happens in front of an audience. You get booed off the court - that's rough. It's not the way you want to move on, but look, I'll take those because they happen quite rarely.

The last time Federer and Nadal met in US soil, it was in 2005 when the Swiss powered past a teenage Nadal, taking revenge for the previous years' upset in their first ever meeting.

Federer went on to insist that despite Novak Djokovic's form, it was Nadal who was his toughest opponent.

My number one rival to me it seems is Rafa, he said. It doesn't matter how many more times I'm going to be playing all the other guys. At the moment, we have that history and nobody can take it away from us. That's why the only thing really missing from us is a US Open match, I guess, and maybe a few other tournaments.

Nadal also spoke about the specialty of their rivalry, saying, For me, it's like a really special thing when we play each other, because we play always really important matches. Tomorrow is a semi-final. Very few times we played in semi-finals of one tournament. The rest of the matches are finals and important for big titles. That makes the rivalry very, very special.