Defending champion Jack Sock beat Australia's Matthew Barton 6-2, 7-6 (5) on Thursday to reach the quarterfinals in the U.S. Men's Clay Court Championship.

The fourth-seeded American won last year at River Oaks for his first ATP Tour title. He will face the winner of a match between seventh-seeded Marcos Baghdatis of Cyprus and Fernando Verdasco of Spain.

Third-seeded Feliciano Lopez of Spain also advanced, beating Lukas Lacko of Slovakia 7-6 (4), 6-1. Lopez will face Tim Smyczek, a 6-2, 6-4 winner over Donald Young in an all-American match.

Fellow Americans John Isner and Sam Querrey reached the quarterfinals Wednesday. The top-seeded Isner will face South Korea's Chung Hyeon, and the fifth-seeded Sam Querrey will play Argentina's Juan Monaco.