Qualifier Michael Russell shocks top seed Mardy Fish

Qualifier Michael Russell stunned top seed Mardy Fish 6-3, 6-1, thrilling his hometown fans as he booked a quarter-final berth at the ATP Tour US Men's Clay Court Championship.

Qualifier Michael Russell stunned top seed Mardy Fish 6-3, 6-1 on Thursday, thrilling his hometown fans as he booked a quarter-final berth at the ATP Tour US Men's Clay Court Championship.

Russell, a 33-year-old who lives in Houston, converted five of his break point chances against the world number nine to triumph in 74 minutes.

It was Russell's first victory over a top-10 opponent in 13 attempts.

Russell booked his first tour-level quarter-final of 2012. He'll face 19-year-old fellow American Ryan Harrison, a 6-2, 6-4 winner over Russian Igor Kunitsyn.

Second-seeded John Isner, playing his opening match after a first-round bye, rallied to beat Horacio Zeballos of Argentina 6-7 (3/7), 6-4, 6-2.

Isner arrived in Houston from Monte Carlo, where he led America to a Davis Cup triumph in France last weekend. He won both of his singles matches in the United States' 3-2 victory in the quarter-finals of the nations tournament.

On Thursday the 26-year-old didn't drop his serve and fired 16 aces in the match lasting two hours and 13 minutes.

Isner, ranked 10th in the world, is seeking a first career clay court title. He next faces defending champion Ryan Sweeting, A 6-3, 6-2 winner over fellow American Bobby Reynolds.

Fifth seed Kevin Anderson of South Africa defeated American Sam Querrey 7-6 (7/3), 3-6, 7-6 (7/2) to reach the quarter-finals.

Anderson, seeking to add a second title of 2012 to the one he captured at Delray Beach last month, next faces fourth-seeded Juan Monaco of Argentina. Monaco defeated Japan's Tatsuma Ito 6-3, 6-3.

Monaco, who claimed his fourth career title at Vina del Mar 2012 and was a semi-finalist at the Miami Masters, is through to his third quarter-final of the year.

The other quarter-final match-up was determined on Wednesday, with third-seeded Feliciano Lopez of Spain advancing to a clash with sixth-seeded Carlos Berlocq of Argentina.