Shelton rallies to eliminate Thompson from Dallas Open

The third-seeded American will face countryman Tommy Paul, the No.2 seed, who beat Dominik Koepfer of Germany 7-5 6-3 in another quarter-final.

In the only all-American quarter-final of the hardcourt event on the SMU campus, unseeded Marcos Giron stunned top seed Frances Tiafoe  6-1, 6-4 to reach the semis here for the second time in three years. 

Giron next faces France's Adrian Mannarino, the fourth seed who has reached the semi-finals without playing a complete match. 

In his first match of the tournament, Mannarino beat James Duckworth when the Australian retired with an injury with the Frenchman leading, 6-2, 3-1.

Shelton saved three break points in the clinching game after doing the same when serving for the second set in a match that didn't even have a break point until the 19th game. 

The 21-year-old, seeking his second ATP Tour title, won with a forehand that hit the baseline on his second match point. 

Shelton lost in five sets to Mannarino in the third round of the Australian Open, after losing to Novak Djokovic in the US Open semi-finals last summer.

"I think any time you can come back and get a win after losing the first set, it's impressive, especially against someone who serves as well as Jordan does," Shelton said.

"He presented so many problems today. I had to problem-solve the whole match." 

After a first set without a break point, the server won the first three points of the tiebreaker, before Thompson's crosscourt winner on a volley with both players at the net for a 3-1 lead. 

Thompson, the No.7 seed, went on to win six straight points for a 6-1 lead and closed out the set two points later when a shot from Shelton sailed long.

Shelton earned the first break points of the match when he got three at 3-3 in the second set.

He converted the third with a forehand winner on the return of serve, then finally closed out the set on his serve when he converted a third set point after saving Thompson's first three break points of the match.

Shelton's best game came at 3-3 in the third set when he had three winners to break Thompson at love. 

The big-serving left-hander topped 225km/h several times while holding off Thompson in the deciding game.

American Marcos Giron has upset the top seed to advance to the semi-finals at the Dallas Open. (Lukas Coch/AAP PHOTOS)

Giron's victory over Tiafoe was his 12th win over a top 20 opponent. 

The 30-year-old, still seeking his first ATP Tour title, lost to Tiafoe 6-2, 6-2 at Indian Wells almost a year ago.

"I was excited to play him again," said Giron, who won the first three points on each of Tiafoe's first two service games on the way to a 5-0 lead.

"Margins are thin. I knew that I was playing well."