Japan all but booked their place in the Women's World Cup last 16 as forwards Hikaru Naomoto and Aoba Fujino scored twice in three minutes in a 2-0 win over Costa Rica on Wednesday.

A sparse crowd of 6,992 spectators saw the match in Dunedin, the lowest attendance so far at the tournament in Australia and New Zealand.

Japan's passage into the knock-out stages from Group C will be confirmed later if Spain beat or draw with Zambia, who the Japanese hammered 5-0 in their opening game.

The 2011 world champions Japan sit top of the group with a perfect six points and have yet to concede a goal.

Spain will surely join them when they face Zambia, the lowest-ranked team at the World Cup, later Wednesday.

"We did what we needed to do and we did it well," said Japan coach Futoshi Ikeda, whose side face the Spaniards in their final group game.

"Some of the players are at a World Cup for the first time but they are really energetic and that makes me so happy.

"The team is overall working as a unit... the atmosphere is good, the energy is good, the staff also bring positive vibes, and I think that is helping the team too."

Costa Rica, well beaten 3-0 by Spain, were floored again by a devastating first-half spell from the Japanese who grabbed two rapid-fire goals.

Outclassed Costa Rica's tournament is realistically already over.

Substitute Gloriana Villalobos managed their one and only shot on target so far at this World Cup with nine minutes left.

Japan took the lead through Naomoto on 25 minutes and they made it 2-0 when they again sliced through Costa Rica's shaky defence moments later.

The 19-year-old Fujino turned her marker, dribbled into the penalty area and powered her shot inside the near post.

The margin of victory would have been much wider were it not for fine saves by Costa Rica goalkeeper Daniela Solera, who had another busy night between the posts but should also have done better with both goals.

The shot-stopper was badly shaken with 25 minutes left after a mid-air collision with Japan replacement Riko Ueki when both players went for a cross.

With their final pool match against Spain to come on Monday, Japan's seven goals so far is a record for them in World Cup group stages.