2003 AIG Japan Open Tennis Championships – Men's singles

Kenneth Carlsen was the defending champion but lost in the third round to Hyung-Taik Lee.

Rainer Schüttler won in the final 7–6(7–5), 6–2 against Sébastien Grosjean.

Seeds
All sixteen seeds received a bye to the second round. • # 🇩🇪 Rainer Schüttler (champion)

• # 🇫🇷 Sébastien Grosjean (final)

• # 🇹🇭 Paradorn Srichaphan (semifinals)

• # 🇨🇿 Jiří Novák (third round)

• # 🇦🇺 Mark Philippoussis (quarterfinals)

• # 🇩🇪 Nicolas Kiefer (third round)

• # 🇰🇷 Hyung-Taik Lee (quarterfinals)

• # 🇺🇸 Jan-Michael Gambill (quarterfinals)

• # Kenneth Carlsen (third round)

• # Mario Ančić (third round)

• # n/a

• # John van Lottum (second round)

• # 🇺🇸 Brian Vahaly (second round)

• # Wesley Moodie (second round)

• # 🇩🇪 Lars Burgsmüller (second round)

• # 🇫🇷 Grégory Carraz (second round)