Houston, April 12:

Argentine Mariano Zabaleta upset defending champion Mardy Fish of the US 7-5, 6-4 on Wednesday to reach the quarter-finals of the US Clay Court Tennis Championships.

Zabaleta, ranked No 156 in the world, now plays last year’s runner-up, Jurgen Melzer of Austria, in the final eight on Friday.

In second-round matches, fifth seed Jurgen Melzer, last year’s runner-up, beat Victor Hanescu of Romania 6-4, 6-2, and Albert Montanes of Spain beat Serbian Dusan Vemic 6-4, 4-6, 6-4.

In the first round, American Alex Kuznetsov beat Lars Burgsmuller of Germany 7-5, 7-5 and Argentine Diego Hartsfield beat Sebastian Grosjean of France 6-4, 6-4.