[2] During the 2017–18 season, he made his debut in all three major forms of the game, first-class, one-day and Twenty20 cricket playing for Cricket Australia XI in the JLT One-Day Cup and tour matches, the Adelaide Strikers in the Big Bash League and South Australia in the Sheffield Shield.
Despite his team's poor performance, he scored 43 runs off 40 balls and was part of a 52-run partnership for the final wicket.
[6] Nielsen was in the Cricket Australia XI squad for the 2017–18 JLT One-Day Cup and played for them in all six matches, one of their standout players in the tournament.
[7] During the 2017–18 Big Bash League season, Nielsen was added to the Adelaide Strikers squad for their semi-final match in order to replace their regular wicket-keeper Alex Carey, who was busy playing for Australia's national team at the time.
[11][12] Nielsen made his Sheffield Shield debut for South Australia a week later when Carey was again called up for national duties.