Opelka hit 39 aces in his victory over IsnerReilly Opelka won a 46-point tie-break - the longest in ATP Tour history - to beat John Isner and reach the final of the Dallas Open.
Second seed Opelka, 24, triumphed 7-6 (9-7) 7-6 (24-22) to set up a meeting with Jenson Brooksby on Sunday.
Opelka, who stands 6ft 11in, finally converted his eighth match point after a run of 26 straight points on serve.
"I lost track. At one point it was 21-all. It's something I've never seen before," he said.
Opelka, 24, and 36-year-old fellow American Isner served a combined 60 aces in a match that lasted two hours three minutes, with neither player facing a break point.
"It just shows how clutch he is," said Opelka. "If it was going to happen, it would have been in this match."
Isner, who is 6ft 10in, was also involved in the longest match of all time, a five-set win over Frenchman Nicolas Mahut at Wimbledon in 2010 that spanned three days and went more than 11 hours.
Eight years later Kevin Anderson beat Isner in a Wimbledon semi-final that lasted six hours 36 minutes.
Fourth seed Brooksby saved four match points in his 6-4 6-7 7-6 semi-final victory over fellow American Marcos Giron.
