Del Potro maintains mastery over Thiem with Acapulco win

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.

(Reuters)

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 millimetres 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 savour 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.

(Reporting by Andrew Both in Cary, North Carolina, editing by Nick Mulvenney)