The youngster is contracted through until the end of the year with Kashiwa Reysol, but the club has expressed a willingness to negotiate with the Bundesliga leaders
The right back was an instrumental part of Reysol's league-winning squad last year, and was also the defensive backbone of the Japan Under-23 team that qualified for the London Olympics.
Dortmund scouts observed Sakai in the Under-23 game against Bahrain on March 14 as well as Reysol's match at Urawa Reds three days later, and were said to be impressed by his physique and technical skills.
Last year's Bundesliga champions are especially excited at the prospect of the 21-year-old joining Shinji Kagawa to create a "Japanese Hotline," and team representatives compared the starlet's abilities to those of Poland international and current Dortmund player Lukasz Piszczek.
"There hasn't been a formal offer, but we're aware of the reports," a Reysol club director told Hochi. "If they approach us, we'll be prepared to negotiate the specifics."
Sakai also received an offer from Brazilian giants Santos in December, but the advances were rejected due to a low transfer fee and the player's desire to wait until after the Olympics to go overseas.