June 21 (Reuters) - Focus on baseball at the 2020 Tokyo Olympics: THE ABSOLUTE BASICS * Baseball returns to the Olympics after being voted off following the 2008 Games in Beijing. * Six teams will be set into two groups for an opening round-robin stage: Israel, Japan, Mexico, South Korea, the United States and one team to be decided in qualifiers later this month. * It gets complicated from there. Second place from each group will square off, as will the third-placed teams. The loser of the third-placed match will be the first eliminated, and the rest of the tournament will be double-knockout. * Games last nine innings, with each team allotted three outs during their turn batting. The team that scores the most runs when batting over the game wins. * A mercy rule in non-medal games will end them if a team is losing by 15 or more runs after at least five innings of batting, or losing by 10 or more runs after at least seven innings. * Each team will have 24 men. * Batters swing with bats, trying to make contact with pitches of sometimes over 100 mph (161 kmh) of a small leather-covered ball. HOW MANY MEDALS? One gold, silver and bronze are at stake, so half the field will take the podium. WHAT HAPPENED IN BEIJING? South Korea won the fifth Olympics baseball tournament, joining three-time winners Cuba and the United States as gold medallists in the sport. WHAT CAN WE EXPECT IN TOKYO? Japan and South Korea have an edge from sourcing players from their top domestic leagues, which are among the most competitive in the world. WHERE IS IT HAPPENING? Fukushima Azuma Baseball Stadium will host one game, despite concerns about it being located near nuclear contamination from a 2011 power plant disaster. All other games are at Yokohama Stadium, which seats 34,000. HOW DID WE GET HERE? Major League Baseball, the U.S. professional league and the world"s premier league, repeatedly has refused to interrupt its annual season to allow players to compete in the Olympics. That has hurt competition and left Olympic officials uninterested in making the sport part of its regular program. WELL FANCY THAT The COVID-19 pandemic has narrowed the competitive field and pushed roster decisions to the last minute. China and Chinese Taipei declined to travel for qualifiers and dropped out. Japan"s manager said in January his roster shortlist included over 180 players, given health and safety uncertainties.