Ballester had a successful junior career and lost the final of the 2018 Boys Amateur Championship at Royal Portrush Golf Club to Conor Gough, 3 and 1.[3][4] In 2019, he won the Spanish U-16 Championship and played for Spain in the European Young Masters, where he won silver at Kunětická Hora in the Czech Republic.[5] He was runner-up at the 2019 Junior Orange Bowl International and the 2020 Desert Amateur in the United States.[6]
At the 2022 Arnold Palmer Cup in Switzerland, he replaced Eugenio Chacarra who turned professional before the event. Ballester won 3.5 of 4 possible points as the international team beat the Americans 33–27.[9]