#1 AJ Styles faced Shinsuke Nakamura in Japan before joining WWE

Before their feud in WWE, AJ Styles and Shinsuke Nakamura collided at New Japan Pro-Wrestling's Wrestle Kingdom 10 pay-per-view event in a dream match for the ages. Styles, who was leading The Bullet Club, faced Nakamura, who was a mainstay on the NJPW roster. The match was ultimately won by Nakamura, who retained his IWGP Intercontinental Championship.
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The match in Japan received rave reviews from all corners of pro-wrestling, and WWE capitalized on the WWE Universe's demand to see this match in a WWE ring. They pulled the trigger on the feud which spanned over the second quarter of 2018.
AJ Styles spoke up for the feud to Sports Illustrated in 2018 before it began:
The only other time we were in the ring together was more than two years ago [in New Japan Pro Wrestling]. We gave people a taste in the “Money in the Bank” match, and that moment let everyone in the back know how special this match could be. The fans responded by asking for this match, and I want to give it to them. Shinsuke won the Royal Rumble, so now we have that opportunity." AJ Styles said (h/t Sports Illustrated)