Hiroki Sakai was born on 12 April 1990 in Kashiwa, Chiba, and came through the youth setup of his hometown club Kashiwa Reysol. He helped Reysol win consecutive promotions and titles — the J2 League in 2010 followed by the J1 League in 2011 — establishing himself as one of the J1's premier right-backs in his early 20s.
In summer 2012 Sakai moved to Germany with Hannover 96 for around €1.2 million. Four years in the Bundesliga were followed by a 2016 free transfer to Olympique Marseille, where he spent five years as Marseille's first-choice right-back across over 150 Ligue 1 matches. In summer 2021 he returned home with Urawa Red Diamonds on a free, immediately making an impact: in 2021–22 he won the Emperor's Cup, and in 2022–23 he lifted the AFC Champions League — Urawa's third continental crown.
In a striking late-career chapter, Sakai joined Auckland FC of Australia's A-League in July 2024. With Auckland in 2024–25 he scored 3 goals and contributed 4 assists across 25 matches, becoming a leadership figure in the new club's first A-League season, and continued his strong form into 2025–26.
For Japan, Sakai earned over 80 caps and appeared at three FIFA World Cups (2014, 2018, 2022). A composed, athletic right-back with stamina, defensive reading and decent crossing range, he is one of Japan's most experienced ever full-backs.