After being dogged by injuries and indifferent form earlier in the season, he is now firing on all cylinders, beating world number one Roger Federer in the Madrid Masters final two weeks ago and again in the third round in Paris.
Frenchman Benoit Paire beat Marcel Granollers 6-1, 6-0 to advance to his first Masters semifinal and will face Roger Federer in the last four.
Fish beat Tim Henman and Tommy Robredo at the Madrid Masters last week before losing to Roger Federer.
He has been in top form recently, beating world No. 1 Novak Djokovic in the semis of the Indian Wells Masters last month before losing to Roger Federer in the final.
World No. 1 Roger Federer begins his bid for a record-equaling 17th Masters 1000 tournament victory against Ecuador's Nicolas Lapentti on Saturday, while third seed and defending champion Andy Murray takes on American Mardy Fish.
CNN: Djokovic dumped out in Miami; Clijsters and Henin progress
Later on Saturday, world No. 1 Roger Federer begins his bid for a record-equaling 17th Masters 1000 tournament victory against Ecuador's Nicolas Lapentti, who made it into the main draw as a lucky loser in qualifying.
CNN: Champion Murray battered by Fish in another Miami upset
Roger Federer, speaking in Shanghai ahead of the season-ending Masters Cup, said officials should come down "very hard" on gambling.
Murray had gone into the US Open with many predicting he could win his first Grand Slam title after a superb run at the Toronto Masters in the build-up, beating Nadal and Roger Federer on his way to winning the tournament.
He is closing on Roger Federer at the top of the rankings, heading into the Paris Masters and the ATP Tour Finals in London next month.
Nadal, whose last title was at the Rome Masters in April, admits Djokovic is the form player in men's tennis after his victory over Roger Federer in the Swiss Indoors final last week.
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