John Isner secures semi-final spot in Houston with Chung Hyeon win

Top seed and former title holder John Isner beat Chung Hyeon 7-6 (7-5) 6-4 to secure a semi-final spot at the ATP clay court tournament in Houston.

Isner, who won the tournament in 2013, saved all seven break points against the rising South Korean star, firing 14 aces and winning 58 per cent of his second serve points.

Although Isner pulled off the win he expects to see teenager Chung continue to rise through the ranks.

"He's only going to get better, he's only going to get stronger, he's going to make all his shots better," Isner said.

"At 19 years old, he's got such an incredible ceiling right now."

The 30-year-old American will take on compatriot Jack Sock for a place in the final, after the defending champion defeated seventh-seed Cypriot Marcos Baghdatis 6-4 6-4.

Sock converted two of his eight break point opportunities and that was enough to extend his Houston winning streak to seven matches.

Another former champion Juan Monaco of Argentina ousted fifth-seed Sam Querrey 6-4 6-4 to book a semi-final against third-seed Feliciano Lopez, a 6-2 6-3 winner over Tim Smyczek.

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