Seiya Sanada
On July 10, 2013, Keiji Mutoh held a press conference to announce the foundation of his new Wrestle-1 promotion, announcing Kai as part of its roster. Kai returned from his injury during the promotion's inaugural event on September 8, when he defeated his surprise opponent Seiya Sanada in a singles match. Following the match, Kai announced that he was going to become the "ace" of Wrestle-1, repeating a claim he had made at the previous day's press conference. Kai and Sanada had a rematch in the main event of Wrestle-1's second show on September 15; this time Sanada was victorious. The two continued working against each other for the rest of the September tour, splitting victories. Kai and Sanada concluded their five match series on October 6 in the main event of Wrestle-1's first-ever show in Korakuen Hall with Kai picking up the win to take the series 3–2. Post-match, he once again proclaimed himself the ace of Wrestle-1. However, despite the seemingly decisive win and Kai announcing that he wanted the match series to end, he was successfully provoked by Sanada into resuming the series, after being pinned by his rival in a tag team match on October 8. Kai and Sanada were finally split away from each other during the November tour, when the singles match series between the two was tied at 4–4 and the tag team match series 1–1. In January 2014, Kai found himself at odds with new on-screen matchmaker Manabu Soya, who claimed that Kai was not worthy of the title of Wrestle-1 ace, after he had been defeated by Minoru Tanaka on January 12, and instead named Seiya Sanada his own ace candidate. Kai then went on a losing streak, which included losses against Tanaka in a rematch, Masakatsu Funaki, and the debuting Yuji Hino.