In the 2018 season, he played 54 games across all competitions, scoring an impressive 13 goals, second in the scoring charts only to forward Yuma Suzuki as Kashima finished the league season in 3rd place. In cup competitions, Doi scored the only goal of the 2018 Emperor's Cup quarter-final against Ventforet Kofu, where Kashima eventually lost in the semi-finals. He also scored in the second leg of the 2018 J.League Cup semi-final against Yokohama F. Marinos, but it wasn't enough to avoid an agonising 4-3 aggregate defeat. The overriding achievement of the year was the 2018 AFC Champions League win – the first time Kashima had won the competition. Doi played a part in 13 of the 14 games on the road to the finals, scoring 3 goals – including at the quarter final stage in a 3-0 victory over Tianjin Quanjian. He also started both games in the finals against Persepolis. The victory meant that Kashima would once again take part in the Club World Cup, where after a 3-2 victory over Guadalajara in which Doi won the man of the match award, they progressed to the semi-final to once again play Real Madrid. Before the semi-final, Doi himself said he was "delighted" to play against Real Madrid again. Madrid ended up as comfortable 3-1 winners, but Doi managed to score a consolation goal, in spite of a VAR review.