KEY BISCAYNE (CBS4) – The lone U.S. player among those left in the Sony Ericsson Open reached the quarterfinals for the first time in his 12-year career by beating Juan Martin del Potro, 7-5, 7-6 (5)

Mardy Fish is 29 and he’ll overtake Andy Roddick and become the top-ranked American for the first time if he defeats No. 6 David Ferrer in the next round.

READ MORE: Florida COVID-19 Hospitalizations Increase

Ferrer topped Marcel Granollers 6-1, 6-2. American John Isner, seeded 30th, lost to Kevin Anderson of South Africa, 6-3, 7-6 (4).

Fish is ranked a career-best 15th. Roddick, his longtime friend, lost his opening match last week and is expected to drop from eighth to about 15th, his lowest ranking since 2002.

Fish hit 38 winners and lost serve only once to outlast del Potro. Both players looked weary at the end of the 2 1/2-hour slugfest in humid, 85-degree sunshine.

While Fish grew up in Florida, the stadium crowd offered noisier support for del Potro, with Argentine fans chanting and singing his name between points.

READ MORE: U.S. Food Exports To Cuba Have Increased By 144 Percent In Last Year

But the match began to swing Fish’s way in the ninth game, when he saved five break points to hold for a 5-4 lead. He needed six set points to break and close the set, doing so only when he clipped the net cord for a lucky winner.

Del Potro didn’t fold, rallying from a break down in the second set. Fish took a 5-love lead in the tiebreaker and whacked a service winner to seal the victory.

Fish lost only eight of 51 points when his first serve was good. The victory sent him to the quarterfinals for only the fourth time in his 47 ATP Masters 1000 tournaments.

Del Potro fell to 20-6 this year. Despite his defeat, the 2009 U.S. Open champion will climb into the top 45 for the first time since beginning a comeback from right wrist surgery last year.

MORE NEWS: It's National Florida Day So Here Are Some Fun Facts About The Sunshine State

(© 2011 CBS Broadcasting Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)