The 19-year-old advanced to the second ATP Tour final of his career, and the second in as many weeks after his run last week in Los Cabos. He will face the winner of fifth seed Taylor Fritz and five-time champion John Isner for a shot at the ATP 250 title.
Nakashima, who improved to 8-3 on the season with the victory, is set to rise into the Top 100 in the FedEx ATP Rankings on Monday. Currently World No. 115, he will be the youngest American in the Top 100.
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To get there, Nakashima had to halt the in-form Ruusuvuori, who toppled the third seed Cameron Norrie and seventh seed Benoit Paire en route to his first ATP Tour semi-final. The American recovered after dropping the opening set, firing 14 aces and breaking serve three times to seal the victory.
“He came out playing really well, and I got off to a slow start,” Nakashima said. “But I just told myself to stick to my tactics out there, and I just served a lot better in the second and third sets. That really helped my ground game a lot. I’m definitely happy to get through this.”
Ruusuvuori came out swinging in the opening set, dominating from the baseline as he created break opportunities in all but one of Nakashima’s five service games and converting on two occasions. But in the second set, the Finn offered up three double faults in the same game to hand Nakashima an early lead, and the 19-year-old never looked back.
Nakashima won 100 per cent (19/19) of his first-serve points in the second set and 92 per cent (12/13) in the third set, effectively keeping his opponent on the back foot. He dropped just two points behind his serve in the second set as he levelled the match, and broke serve twice in the third set to seal the victory after an hour and 55 minutes.