(Reuters) - Argentine Juan Martin del Potro maintained his perfect career record against Dominic Thiem on Thursday with a popular 6-2 7-6(7) victory over the Austrian in their Acapulco Open quarter-final in Mexico.

The former U.S. Open champion raced through the first set against the world number six but had to battle much harder to secure the win in a tight second stanza on the outdoor blue hardcourt.

The victory came in the tiebreak on a Thiem double fault, which was confirmed after the third seed, with nothing to lose, challenged the call.

Hawkeye revealed the ball had been a few millimeters long, sending the enthusiastic pro-del Potro crowd into wild cheering and applause.

Del Potro, who improved his head-to-head record against Thiem to 4-0, will have little time to savor his victory as he prepares for Friday’s semi-final against second seed Alexander Zverev.

German Zverev, the highest seed still standing after the pre-tournament withdrawal of Rafa Nadal, earlier dusted American Ryan Harrison 6-4 6-1.

Kevin Anderson also reached the last four after serving 18 aces and saving six break points to edge South Korean young gun Chung Hyeon 7-6(5) 6-4 in a quarter-final that stretched nearly two hours.

Standing in the South African’s path to the final will be 21-year-old unseeded American Jared Donaldson, who won 100 percent of his first serve points in a 6-3 6-1 thrashing of Spaniard Feliciano Lopez.