Petrova produced a good first round to open up a three-point lead as Fernandes struggled to settle in the early exchanges.
The Kiwi made more of a fight of it in the second with some decent combinations, but still fell another point further behind.
The fight stepped up a gear in the third round as Fernades pressed forward, but Petrova's movement was keeping her out of trouble.
The Bulgarian was using her superior footwork to outfox her opponent, while throwing out a number of good shots to rack up the points.
She ran out a comfortable 23-11 winner.