KEY BISCAYNE, Fla. — Mardy Fish might leave Key Biscayne as the top-ranked American in men's tennis, a prospect that leaves him unconvinced.

KEY BISCAYNE, Fla. — Mardy Fish might leave Key Biscayne as the top-ranked American in men's tennis, a prospect that leaves him unconvinced.

"I wouldn't be the No. 1 American really," said Fish, who beat No. 17 seed Richard Gasquet 6-4, 6-3 in the third round of the Sony Ericsson Open on Sunday.

Fish is ranked a career-best 15th, and if he wins two more rounds he'll overtake longtime friend Andy Roddick in the next rankings. Roddick lost his opening match Saturday and is expected to drop from eighth to about 15th, the lowest he has been ranked since 2002.

Roddick is a five-time Grand Slam finalist who won the 2003 U.S. Open and finished that year ranked No. 1. Fish's record in major events is barely above .500, and he has made the top 20 in the year-end rankings only twice.

But at 29, Fish is in the best condition of his career. That showed against Gasquet in the midday sun on an 87-degree afternoon.

"Playing out there today, you can obviously see why you need to be fit to win these matches," Fish said.

Also advancing was No. 30-seeded John Isner, who won an all-American matchup against qualifier Alex Bogomolov Jr., 6-2, 7-6 (4). Bogomolov upset Roddick in the second round, giving Isner a less daunting path to the quarterfinals.

Fish's next opponent will be Juan Martin del Potro, who is gaining momentum in his comeback from right wrist surgery last May. The big Argentine earned his first win this year over a top-10 player by beating No. 4 Robin Soderling 6-3, 6-2.

Del Potro, ranked 484th in February, will climb back into the top 45 after the tournament.

"It's a long road to come back to being at the top again," the 2009 U.S. Open champion said. "I'm improving very, very slowly."

In women's play, No. 2-seeded Kim Clijsters earned her 500th career victory by beating Maria Jose Martinez Sanchez, 6-4, 4-6, 6-3.

Reigning French Open champion Francesca Schiavone, seeded fifth, edged Lourdes Dominguez Lino 6-4, 7-6 (2). No. 9 Agnieszka Radwanska beat No. 24 Maria Kirilenko 7-6 (2), 6-3.