The match-winner for India came in the 13th minute from Rani Rampal, who capitalised on a rebound from the Japanese goalkeeper after the initial shot from Vandana Katariya in open play was blocked.
But the star of the day for India was Savita, who firmly stood between repeated Japanese raids and the Indian goal, often diving to either side to block firm shots.
In the last quarter itself, Japan forced five penalty corners and laid a seize on the Indian circle, but could not get a shot past Savita.
Indian women's lone Olympic appearance was at Moscow in 1980. Effectively, India's fifth spot here should secure them an Olympic Games ticket when the countdown to allocate spots takes place.
Three spots each are on offer at both the World League Semifinal competitions, here at Antwerp and in the just-concluded event at Valencia.
With five continental event champions being allowed another route, there are likely to be several dual qualifications. In turn, that can open up Olympic spots for nations which do not make the top four of the two World League Semifinals.
An African champion could be the only one not to have featured among the top four teams at these two World League Semifinals, opening up more places, even if Olympic hosts Brazil are given a leeway to make the grade.
Both in the men and women's events, Brazil have been allowed a leeway of making the Olympics if they can achieve the fifth place in the Pan American Games. In the women's event, Brazil are only a reserve team in case there are any withdrawals from the Pan American Games that start in Canada in two weeks.
