Defending champion Kei Nishikori had to come from behind to beat Alex Bogomolov at the US Indoors in Memphis.
The world No 16 saved all four break points he faced in the final set to come through 3-6 6-3 6-2 against the Russian.
Bogomolov, rated 80 spots below Nishikori and still seeking his first ATP title at 30, broke Nishikori's first two service games for a 4-0 lead.
The Japanese broke back in the seventh game but Bogomolov held in the ninth to take the first set.
The second set featured two early exchanges of breaks before Nishikori held for a 4-3 edge, then broke the Russian and held again to force a third set.
Nishikori broke for a 2-1 edge and again to seize command before fighting off the Russian's final break chance in the last game.
"Obviously, he was playing really well, I thought," Nishikori said. "He was hitting ball really flat and deep and wasn't making mistakes, especially from the beginning he was very aggressive. I wasn't maybe playing 100 percent."
He will take on American Michael Russell in the semi-finals after he overcame Australian third seed Lleyton Hewitt 6-3 7-6.
It means 35-year-old Russell will play just his third semi-final on the ATP tour, eventually prevailing despite double-faulting twice when trying to serve it out at 5-4 and again dropping match points in the tie-breaker.
Fourth-seeded Lu Yen-Hsun of Taiwan beat American qualifier Alex Kuznetsov 6-1 6-4 to set-up the second semi-final with Croatia's Ivo Karlovic, after his 7-6 6-7 6-3 win over American Jack Sock.