Rai's first experience of professional tennis was being given a wild card into qualifying for the 2018 ASB Classic in Auckland, where he was well-beaten by Taro Daniel in the first round. He played his first senior ITF tournament in Kampala, Uganda, in May, qualifying for the main draw in singles, where he reached the second round. The following week, at the same venue, he was given a wild card into both doubles and the main draw for singles, and reached the quarter-finals of both.
He reached his first doubles semi-final in China in July, but the highlight of his year to that date would come in September, when he made his Davis Cup debut for New Zealand, partnering Artem Sitak to win their doubles rubber against South Korea, giving Rai a perfect start to his senior international representative career. His first ITF doubles final came in Hua Hin, Thailand, in October, where he and Karunuday Singh lost in a match tie-break to the top seeds, Francis Casey Alcantara and Sonchat Ratiwatana. In singles at the same tournament, he progressed past the quarter-finals for the first time, going all the way to take the title over Manish Sureshkumar in three sets.