The 24-year-old became the first male player from Asia to advance to any grand slam final after beating Novak Djokovic before a disbelieving crowd at the Arthur Ashe Stadium on Saturday.
The surprise win over the world number one has lifted sales at sponsor Fast Retailing, owner of the Uniqlo casualwear brand. Even a non-sponsor like racket maker Yonex Co saw its shares reach multi-year highs.
Uniqlo's line of Kei Nishikori polo shirts with its red-and-white logo patch has sold out in Japan. The only item left in its online store for Nishikori's U.S. Open line on Monday were a pair of white track pants.
Swiss watch brand TAG Heuer, owned by LVMH, is also a sponsor.
Japanese pay-TV operator Wowow Inc, which caught national broadcasters flat-footed by securing exclusive rights to air all matches over the two-week tournament, is also reaping rewards from Nishikori's victory.
"We've had more than 10 times the number of queries for our service compared to the same time last year," Wowow spokesman Osamu Sakiyama said. "We've never seen anything like this."
He declined to disclose the number of sign-ups that Wowow has had over the past week, when Nishikori made headlines by becoming the first Japanese to reach the quarter-finals and semi-finals in almost a century. Subscription data for September will be announced in early October.
Shares of Wowow touched a 13-year high of 4,880 yen on Monday before closing up 7.1 percent at 4,700 yen.
Another winner on the Tokyo Stock Exchange is Yonex as investors predicted a surge in sales on the back of soaring interest in the sport.
Yonex's shares ended up 1,080 yen at an eight-year high despite having no sponsorship deal with Nishikori, who uses a Wilson. They have gained 32 percent since the U.S. Open kicked off.
In contrast, shares of sponsor Nissin Foods Holdings have lost 6 percent over the same period, with little immediate impact seen on its sales.
"We have a long-term contract with him and we're more after the positive impact on our corporate and brand image," Nissin spokesman Masashi Kanaya said. "I think his performance has been great for our image, particularly overseas."
Nishikori will face Croatia's Marin Cilic for the men's title on Monday.
(Reporting by Chang-Ran Kim; Editing by Ryan Woo)
