Sugita and Naomi Osaka represented Japan in the mixed-gender 2018 Hopman Cup. He lost his singles match to Roger Federer 6–4, 6–3, but his leaping overhead smash delighted the fans and was included in most compilation clips of early 2018 season highlights. At the 2018 Australian Open Sugita earned his first win over a Top 10 player by defeating world number 9 Jack Sock in four sets. He lost in the next round to Ivo Karlović. In February he lost in the first round in Rotterdam to eventual losing finalist Grigor Dimitrov. In March he was defeated by Horacio Zeballos in the first round at Indian Wells, and by Robin Haase in the first round at Miami. In April he lost in the first round to Jan-Lennard Struff at Monte-Carlo, Guillermo Garcia-Lopez at Barcelona, and Yannick Maden in Munich. In May he lost in the first round to Philipp Kohlschreiber in Madrid and to Ryan Harrison in Rome. In June Sugita lost in the first round of the French Open to Horacio Zeballos. Later in the month he lost in the second round in s-Hertogenbosch to Marius Copil; and the next week at Halle beat world #7 Dominic Thiem in straight sets, in what he said was “my best match in my career,” before losing in the quarter-finals to Denis Kudla. At Antalya, Turkey, Sugita lost in the second round to Pierre-Hugues Herbert. In July he lost at Wimbledon to Bradley Klahn in the first round. In August he lost in the first round to Vincent Millot at Washington; lost in the first round to Ilya Ivashka at Toronto; lost in the first round of qualifying at Cincinnati; lost in the first round to Leonardo Mayer in Winston-Salem; and lost in the first round of the US Open to Richard Gasquet. Sugita started the week after the US Open with a record of 8 wins and 22 losses for the year, and his ranking was #98 in the world. In October he received a wild card entry into the Tokyo Open, and lost in the first round to countryman Kei Nishikori.