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DALLAS (AP) — The semifinals of the inaugural ATP Dallas Open will feature four Americans.

Seventh-seeded Marcos Giron upended No. 1 seed Taylor Fritz — who is also his practice partner — 6-4, 3-6, 7-6(3) on Friday to advance. The rest of the semifinals field includes second-seeded Reilly Opelka, third-seeded John Isner and No. 4 Jenson Brooksby. In Saturday's semifinals, Giron will face Brooksby, while the 6-foot-11 Opelka and 6-10 Isner will take the game to great heights.

Dallas resident Isner made quick work of ending the run of qualifier Vasek Pospisil of Canada 6-4, 6-4. Isner will be playing in both the singles and doubles semifinals on Saturday. Opelka didn't waste time, either, beating fifth-seed Adrian Mannarino of France 6-3, 6-1. And Brooksby dispatched Australian Jordan Thompson 6-3, 6-0.

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Having four Americans in the semis is rare; the last time it happened at an ATP event was in 2004 in San Jose, California.

Giron battled back after losing the second set and falling behind 4-2 in the third set.

"When I was down a break in the third, we got a change of balls, and I was able to kind of take advantage of that," Giron said. "Once I was able to sneak out a break, my level improved." By defeating the 19th-ranked player in the world, No. 70 Giron scored his fifth career win in 18 chances against top 20 players to reach his second ATP semifinal.

Fritz was playing in his first tournament as the top seed.

"I've practiced with Fritz so much," Giron said. "We're both based in LA and we both train in Carson (California) so we know each other's game well."

Meanwhile, Isner put 81% of his first serves in play and had 19 aces with the stadium timer recording speeds up to 133 miles per hour.

"This is a good serving court," Isner said. "It's indoors. There's no wind, no sun. It's the same thing every single time no matter what time of the day you're playing." He acknowledged that it had been important to win quickly to stay better rested for his doubleheader on Saturday.

"I know I'll get to bed earlier tonight," Isner said. "Things went my way pretty early. I did have a feeling that I was going to play better. I'm just feeling the ball better."

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If history repeats, the clash of the big men could be an epic battle. Opelka has won the last three of his four ATP meetings with Isner. In the four matches, 11 of the 13 sets went to tiebreaks. After taking a 3-2 lead in the first set with a service break, Brooksby fell behind love-40 but rallied to win that game.

"I think that was a turning point," Brooksby said. "I loosened up a little bit and got more confidence and belief after that."

Thompson won only one more game in the match.

"It's good for me to get a lead like that. You loosen up, you can play a little freer. Playing the right way heading into the (semifinal) match," Brooksby said. "Also, a little less time on court just helps going into the next match."

In quarterfinal doubles, Americans Sam Querrey and Jackson Withrow defeated American Nathaniel Lammons and Andre Goransson of Sweden 7-6(4), 7-6(3). Lloyd Glasspool of Great Britain and Harri Heliovaara of Finland held off Mexicans Hans Hach Verdugo and Miguel Angel Reyes-Varela 6-1, 7-6(3).

Querrey and Withrow will take on top-seeded Marcelo Arevalo of El Salvador and Jean-Juliern Rojer of the Netherlands, the only remaining seeded pair. Glasspool and Heliovaara will meet Isner and fellow American Jack Sock in Saturday's semifinals.