Pace spearhead Elysse Perry had missed the entire Super Six stage with an ankle injury and was struggling to bowl her first ball.
Fields said it was tough to leave out the 17-year-old Holly Ferling, who picked up nine wickets in four matches, for Perry in the final.
"That was a really tough call when we went to choose the team, I knew all 15 players could win the game for us. But to leave out Holly (Ferling) was a tough decision. Elysse, she came through today.
"I must admit my heart sank a bit (when she couldn't get her run up right twice) and I thought oh no. But she came with the goods and she is a top level player," she said.
Fields said the side had prepared hard for this World Cup after a disappointing show in 2009 when they were the host nation.
"I was lucky to be part of the 2009 campaign in Sydney. It was a very disappointing campaign for the team at that point. It was a home tournament and we performed poorly," she said.
"In the last four years, there has been a big effort by not only the playing group but the coaching staff and everyone from Cricket Australia.
"So when we turned up for this World Cup, we were in the best shape possible. To everyone's credit we have done that and proved why we are the number one team in the world," she said.