In May 2018, Tubelis won the 2017–18 Adidas Next Generation Tournament (ANGT) with the U18 team of Lietuvos rytas Vilnius.[1] On 18 October 2018, he signed a long-term contract with Rytas Vilnius.[2] In the 2018–19 season, he played for Perlas in the National Basketball League (NKL), the second-tier league of Lithuania. He averaged 9.6 points, 3.7 rebounds and 1.4 steals per game.[3] In February 2019, Tubelis played for Rytas' U18 team at ANGT Kaunas, where he was named to the all-tournament team after averaging 15.3 points, 6.5 rebounds, three steals and 2.8 blocks per game.[4] In June 2019, Tubelis took part in the Basketball Without Borders Europe Camp in Riga.[5]
In 2019–20, Tubelis split playing time between Perlas and Rytas' senior team, emerging as one of the top players in the NKL.[6] He debuted for the senior team in a 6 October 2019 win over Juventus Utena.[3] Tubelis scored an LKL season-high 11 points in a victory over Šiauliai on 1 February 2020.[6] At ANGT Kaunas in February 2020, he averaged 20.5 points and 12.5 rebounds, leading Rytas' U18 team to first place and earning tournament most valuable player honors.[7] Later that month, Tubelis participated in the Basketball Without Borders Global Camp at 2020 NBA All-Star Weekend in Chicago.[8] In 25 NKL games, he averaged 16.4 points, 6.2 rebounds, 1.5 steals and 1.5 blocks per game.[9]
On 2 February 2023, Tubelis posted a career-high 40 points and nine rebounds in an 91–76 win over Oregon.[12] As a freshman, he averaged 12.2 points and 7.1 rebounds per game, and was named to the All-Pac-12 honorable mention and All-Freshman Team.[13] Tubelis was named first-team All-Pac-12 as a sophomore and a junior.[14] He declared for the NBA draft, but was not widely projected to be selected.[15] According to Seth Davis of The Athletic, there were questions on his ability to switch onto smaller players away from the basket.[16]ArizonaSports.com wrote that his slow footspeed could be a concern.[17]
Tubelis has a twin brother, Tautvilas, who also currently plays basketball for the Arizona Wildcats and has represented Lithuania at the international level.[10] Tubelis' first name, Ąžuolas, translates to oak in Lithuanian and was given to him by his grandfather.[26]