(Reuters) - LeBron James opted not to play for Team USA at the Rio Games but the greatest player of his generation said he misses the Olympic stage and has left the door open to joining the national squad at the 2020 Tokyo Games.

James, who decided to skip what would have been his fourth Olympics after leading the Cleveland Cavaliers to an NBA title in June, said in an ESPN interview that he is keeping an eye on his team mates at the Rio Games.

"Every time I watch 'em, I wish I was out there," double Olympic champion James said. "I did not retire from Team USA. I just did not play this summer. So I still left the door open."

The United States advanced to the semi-finals of the men's basketball tournament at the Rio Olympics with a 105-78 win over Argentina on Wednesday.

After leading Cleveland to an historic comeback from a 3-1 series deficit in the best-of-seven NBA Finals to win the title, James, who has reached the championship round in each of the last six seasons, said he needed rest.

James joined USA Basketball for the 2004 Athens Olympics where the Americans took home a bronze medal. He returned and won gold medals with Team USA in 2008 and 2012.

Should James come back to play for the national team, the four-times National Basketball Association Most Valuable Player would be 35 at the 2020 Tokyo Games.

(Reporting by Larry Fine in New York; Editing by Frank Pingue)