Kruger was involved in provincial rugby since primary school level, representing the SWD Eagles at the Under-13 Craven Week competition in 2006. He was also selected to represent the SWD Eagles at Under-16 level, playing at the Grant Khomo Week in 2009, and at Under-18 level, playing at South Africa's premier high schoolrugby union tournament, the Craven Week, in 2011.
After high school, Kruger moved to Bloemfontein to join the. He made seven starts for the team during the 2012 Under-19 Provincial Championship, scoring tries in their matches against the and the. Free State lost out on a semi-final spot in the competition, however, finishing in sixth spot. In 2013, he made the step up to Under-21 level and played in ten of the team's matches during the 2013 Under-21 Provincial Championship. He again scored two tries – against eventual champions in an 18–16 victory in the opening match of their season and against – as his side once again missed out on the semi-finals by finishing fifth. He started the 2014 season by representing university side in the 2014 Varsity Cup. He made four appearances and scored against in a 52–13 win as UFS Shimlas finished the competition in fifth spot. He started nine matches for in the 2014 Under-21 Provincial Championship and scored tries against and in both of their matches against the. His side finished fourth to qualify for the semi-final, where they were beaten by Western Province. Kruger started seven of the ' matches during the 2015 Varsity Cup competition, helping Shimlas finish top of the log and qualify for a home semi-final. He featured in their semi-final match against two-time champions where Shimlas secured a 21–10 victory to help them qualify for their first ever final. He also started the final against the as Shimlas ran out comfortable 63–33 winners to win their first ever Varsity Cuptitle.
He was included in the senior squad for the first time for the 2015 Currie Cup Premier Division and was initially named on the bench for their Round Two match against, but subsequently replaced by loose-forward Tienie Burger. He was again named on the bench for their Round Four match against the same opposition, but failed to make an appearance. He eventually made his first class debut on 19 September 2015 as he played off the bench in the Free State Cheetahs' 44–24 victory over. He made his first start a week later in their 31–73 defeat to the.
He is the younger brother of number eightDe Wet Kruger. The both played for the Free State Cheetahs in the 2015 Currie Cup; Tertius made his first class debut one week before De Wet and the two brothers featured in a match together when De Wet made his debut in their defeat to the.
