Kei Nishikori, played nearly mistake-free tennis to move into his 11th ATP quarter-inal of the season by beating 16th-seeded Leonardo Mayer of Argentina 6-4, 6-4 at the Citi Open in Washington DC.

Kei Nishikori, played nearly mistake-free tennis to move into his 11th ATP quarter-inal of the season by beating 16th-seeded Leonardo Mayer of Argentina 6-4, 6-4 at the Citi Open in Washington DC.

The second-seeded Nishikori made only seven unforced errors, half as many winners as he produced in the one-and-a-half-hour victory.

Nishikori meets Sam Groth in the semi finals. Groth delivered 20 aces in a 6-3, 6-4 victory over seventh-seeded Feliciano Lopez.

Sam Querrey gave U.S. Open champion Marin Cilic all he could handle in the first set - until the tiebreaker. In the second set, too - until the tiebreaker.

Getting his hard-court game in gear a few weeks before defending his first Grand Slam title, Cilic edged the 13th-seeded Querrey 7-6 (2), 7-6 (3) to reach the quarter finals.

Jack Sock and Steve Johnson set up an all-American quarter final by eliminating seeded opponents.

Sock came back to beat No. 4 Richard Gasquet of France 6-7 (4), 7-6 (5), 6-4, while Johnson got past No. 6 Grigor Dimitrov of Bulgaria 6-3, 1-6, 6-3.