HOUSTON: Argentina's Juan Ignacio Chela beat defending champion Lleyton Hewitt of Australia 6-4, 6-3 to reach the semi-finals of the US Men's Clay Court Championships.

Hewitt, seeded fourth, was playing his first tournament since surgery on his right hip on January 29.

The Aussie veteran fell behind 5-0 in the second set, made a run back with breaks in the sixth and eighth games but stalled as he was broken in the ninth game of the set.

Chela, ranked 84th in the world, next faces sixth-seeded compatriot Horacio Zeballos in the semi-finals.

Zeballos upset top-seeded Fernando Gonzalez of Chile 6-4, 6-4.

Zeballos, the 2009 ATP newcomer of the year, sealed the victory with an ace on his first match point.

He had broken the world number 11 in the opening game of the second set, and fended off a series of break points to make it stand up.

"Zeballos played a good match and I didn't play my best tennis," Gonzalez said.

Third-seeded Sam Querrey of the United States also advanced, downing Chile's Nicolas Massu 6-3, 3-6, 6-4.

Querrey will face American Wayne Odesnik, a 6-4, 6-1 winner over Belgium's Xavier Malisse.

Odesnik is playing even as he is being investigated by the International Tennis Federation and possibly facing a two-year ban.

The American pleaded guilty in Australia to possessing banned human growth hormone.

The ITF said earlier this week that Odesnik was free to play pending its investigation into the matter.

"I would refuse to lose to that guy," Querrey said.

Despite pleading guilty in Brisbane Magistrates Court to importing HGH, Odesnik has denied taking HGH or any other banned performance enhancer.

Odesnik, who has declined comment on his case this week, responded: "We'll see what happens tomorrow."