Hege Riise praised Team GB's unity after in-form striker Ellen White struck again to book their spot in the last eight at the Olympics.
The Manchester City striker grabbed her third goal in two games to seal a cagey 1-0 win over hosts Japan in Group E.
Team GB dominated the second half but were forced to be patient against an organised Japan team.
They sit top of Group E with six points, ahead of Canada who also beat Chile 2-1 on Saturday. Team GB play Canada for the right to finish top in Kashima on Tuesday.
Riise said: "In the first half, we didn't look like ourselves, we didn't have the speed of play we wanted. We came out as a different team with an attitude and togetherness and we all committed to the last 45 minutes.
"In the second half we changed formation a little bit and controlled it much better.
"We are all happy, it puts us in a good position and we're able to rest players and recover well. The connections between the players will get better and better each game.
"We were surprised Japan made changes but they had momentum in the first half."
A forgettable first half saw Mina Tanaka drive wide after 32 minutes in the only scare for Team GB despite disciplined Japan slowly taking control.
Two tame Lauren Hemp headers which dropped wide were Team GB's best chances during a cautious opening 45 minutes.
Riise's side tried to pick the pace up after the break but were stifled by the hosts, although Japan failed to make a telling impact themselves.
Steph Houghton volleyed over after 63 minutes and Nikita Parris curled wide soon after as Team GB dominated the ball.
But White soon capitalised on a defensive error to give Team GB the win they deserved with 16 minutes left.
Lucy Bronze's cross from the right should have been dealt with by the Japan defence but White stole in ahead of goalkeeper Ayaka Yamashita to glance the ball in.
Caroline Weir was close to adding a second when her free kick dropped a yard over with a minute left.
Japan boss Asako Takakura said: "We played against a great team, they were a great opposition. We knew coming in to the game it was not going to be easy so we had discussions prior to the game.
"We talked about what we had to do with our defence and we had to be very patient.
"In the first half we executed as we had planned, in the second half the plan was to make more offensive moves but our opponent was very strong and were able to go forward."
