Cuba have won three of the five Olympic baseball gold medals on offer since it came onto the programme in 1992 but failed to qualify for Tokyo after defections by several top players.
That has opened up the field in baseball's first appearance at the Olympics since 2008, when South Korea denied the Cubans a fourth gold.
Major League Baseball (MLB) in the United States has barred participation by its players, including batting leaders Shohei Ohtani and Fernando Tatis Jr.
The restriction -- aimed at avoiding a mid-season stoppage -- has robbed all six Olympics teams of the world's elite players but has had more of an impact on the United States, Dominican Republic, Mexico and Israel.
Nippon Professional Baseball (NPB) in Japan and the Korea Baseball Organization (KBO) will pause for the Olympics.
While the contrasting stance of the leagues is not new, South Korea and Japan come into the Olympics with their players having largely been less affected by COVID-19 lockdowns.
NPB played 120 games last year, and South Korea its full 144-game slate. MLB held a 60-game season, well below its typical 162. U.S. minor leagues and the Mexican League cancelled last year.
"NPB is one of the most excellent leagues in the world," Japan's manager Atsunori Inaba told Reuters. "Daily experience in games at high level will surely make the players" their best.
Foreign spectators are banned because of COVID-19 concerns, ensuring Japan will have stout home backing.
Not having to travel also helps.
Mexico plan to practice at 3 a.m. in the five days before their 14-hour flight to Tokyo, borrowing a preparation technique from boxers who compete overseas.
Familiarity with a more pitcher-friendly ball than is used in the West may also aid Japan, who will also benefit from the fact their players know each other so well: five of their 24 players hail from one NPB team alone.
Inaba's roster includes the NPB's home run leader and some of its toughest pitchers, including Masahiro Tanaka, but he declined to declare Japan a favourite, saying each country could contend.
South Korea has the KBO's top batters by average and on-base plus slugging percentage, though a young pitching staff.
Israel is in an Olympics team sport for the first since soccer in 1976. Its roster includes MLB free agents and even an MLB coach.
Mexico manager Benjamin Gil is looking for big contributions from minor leaguers including pitcher Hector Velazquez and one-time MLB standout Adrian Gonzalez, who currently plays for Gil's Guadalajara team.
"He is the most productive first baseman in the league," Gil told Reuters. "He has erased any doubt that he can still play."
The U.S. and Dominican rosters are still unclear, with potential players bouncing between MLB and the minors. Reporting by Paresh Dave; Editing by Peter Rutherford