He moved to Potchefstroom at the start of 2015, joining their provincial team, the. He made twelve starts for the team in the 2015 Under-19 Provincial Championship, starting five matches at inside centre, five at outside centre and two at fullback. He scored tries in matches against, and and also kicked fourteen conversions and three penalties for a total of 52 points, the second-highest points tally for a Leopards player and tenth overall in Group A of the competition. Janse van Rensburg represented the in the 2016 Varsity Cup, where he made history in their opening match against the ; the Varsity Cup adopted a points system whereby tries that originate within a team's own half were worth nine points, and Janse van Rensburg scored the first nine-point try in the competition after just nine minutes. He scored a second try in the match, a 38–15 victory for NWU Pukke, and two more in matches against the and to finish as his side's second-highest try scorer. In March 2016, Janse van Rensburg was included in a South Africa Under-20 training squad, but wasn't initially included in the final South Africa Under-20 squad for the 2016 World Rugby Under 20 Championship tournament to be held in Manchester, England. However, following injuries to S'busiso Nkosi and Embrose Papier during the tournament, he flew out to meet up with the rest of the team. He came on as a replacement in their final match of the tournament, the third-place play-off against Argentina. Argentina beat South Africa for the second time, convincingly winning 49–19 to condemn South Africa to fourth place in the competition. Shortly before his involvement in the World Rugby Under 20 Championship, he also made his first class debut in South African domestic rugby. He started their Currie Cup qualification match against the, helping them to a 26–24 victory. After another start against the, he dropped to the bench for their match against the, but came on to score his first points in first class rugby, converting two late tries. He started five more matches in the fly-half position before his short spell with the South Africa Under-20 team, without scoring any points. On his return, he made two more starts; in the first of those, he scored a total 13 points – including his first senior try – in his side's 23–26 defeat to a, and a week later, he scored a try and five conversions in a 54–14 victory over the.
