De Minaur commenced the year at the Brisbane International after receiving a wildcard into the main draw. He defeated American Steve Johnson in straight sets, and then scored the biggest win of his career to date by beating world number 24 Milos Raonic in straight sets. He then defeated qualifier Michael Mmoh in the quarterfinal before losing to Ryan Harrison in the semifinal. De Minaur is the lowest ranked player and the youngest to reach the semifinals of the men's draw in the Brisbane International's 10-year history. De Minaur received a special exempt spot in the main draw of the Sydney event, where he eliminated consecutively Fernando Verdasco, Damir Džumhur and Feliciano López to reach his second ATP tour semifinal, a week after he reached his first in Brisbane. De Minaur became the youngest player to play in two consecutive ATP semi-finals since Rafael Nadal in 2005. He beat Frenchman Benoît Paire in the semifinal to meet Daniil Medvedev in the final. De Minaur lost the final in three sets, having won the opener. At the 2018 Australian Open, de Minaur lost in the first round to Tomáš Berdych, but took a set off of the 19th seed. In February, De Minaur made his Davis Cup debut for Australia, losing to world number 5 Alexander Zverev from Germany in 5 sets. In March, De Minaur lost in second round of Indian Wells to eventual champion Juan Martín del Potro before qualifying for and losing in the first round of Miami Masters. In April, De Minaur reached his third Challenger final in Alicante.