Third-seeded Isner, who needed almost three hours to beat fellow-American James Blake in three tough sets on Wednesday, faced little resistance from Lu. Isner won in 56 minutes, earning quick relief from the Atlanta heat.

“It’s good to get off the court quickly,” Isner said. “It’s what I didn’t do my first round, so I’ll definitely be fresh tomorrow.”

Gilles Muller of Luxembourg produced 14 aces to upset No. 2 seed Kevin Anderson of South Africa 7-6 (3), 7-6 (3) in the day’s first match. Muller advanced to his first ATP semifinal since 2005 and will play Isner on Saturday.

Top-seeded Mardy Fish of the United States advanced with a 6-4, 6-3 win over India’s Somdev Devvarman.

With dark clouds looming, Fish said he faltered late when rushing to complete the match.

“Obviously those things were pretty ominous,” Fish said of the clouds. “I just rushed myself a little bit there. ... It got a little hairy there. I was lucky to get off.”

Ryan Harrison and Rajeev Ram were to play in the last quarterfinal, with the winner playing Fish on Saturday.