Fish will now play the world No 2 Novak Djokovic at Key Biscayne after the Serb beat South African Kevin Anderson 6-4, 6-2 for his 24th successive win, a run stretching back to last year.
Fish, currently ranked 15th, will formally overtake his friend and the world No 8 Roddick when the ATP world rankings are updated next week. Roddick, struggling with a bronchial infection, made a shock early exit in the defence of his Miami title in his first match, losing to Uruguayan Pablo Cuevas on Saturday
"I don't think I will ever feel I am the top-ranked American, Andy has had a pretty good career and he has always been top dog in my generation," Fish said of his former high-school team-mate. "I've never been in this position before. Obviously it's very humbling. This is a pretty big country and has a lot of people playing tennis. So I'll try to put it into perspective, and I will."
Fish had a surprisingly easy passage to the last four, with the normally solid Ferrer fading fast in the second set in the Florida heat. The Spaniard said he had not felt well. "I had stomach problems all match. I went to the court, and when I ran to rallies I had a lot of pain in my stomach. In the second set, I couldn't run any more," he said.
Djokovic was pumped up throughout his win over Anderson, but he had to work hard against the big-serving South African, who produced four aces. Nonetheless, the Serb produced the big shots when it mattered most – breaking early in both sets.
Roger Federer, the last man to beat Djokovic in London in November, eased into the semi-finals – and a showdown with Rafael Nadal – whose oppenent Gilles Simon retired with a stiff neck after losing the first three games of their quarter-final.
Maria Sharapova overcame a slow start by winning 11 consecutive games to reach the women's final last night, beating Andrea Petkovic of Germany 3-6, 6-0, 6-2.
Earlier, Kim Clijsters admitted she ran out of "fighting spirit" as she lost to Victoria Azarenka in the quarter-finals. The defending champion was beaten 6-3, 6-3 by the eighth seed. She said: "I just didn't feel good. Just mentally, physically, I didn't feel right."
