Sakai, a student of Waseda University, competed in the 1970 Summer Universiade, which was held in Turin, Italy. He and Jun Kamiwazumi won the doubles gold medal for Japan.

When the Japanese Davis Cup side upset Australia in 1971, Sakai played a large role, winning both of his matches, over Colin Dibley and the deciding fifth rubber against John Cooper. Sakai made the third round of the 1971 US Open, beating Vitas Gerulaitis and Ross Case.

In 1972, Sakai was the inaugural winner of the Japan Open Tennis Championships and a semi-finalist in the U.S. Men's Clay Court Championships.

At Wimbledon in 1973, Sakai was eliminated in the third round by Ilie Năstase, having earlier secured wins over Frew McMillan and Hans Kary. In Osaka later that year, he beat Zeljko Franulovic in the semi-finals to make it into his first Grand Prix final, which he lost to Ken Rosewall. In 1973 he also managed to defeat John Newcombe in a Davis Cup match.