Biles marked the occasion by becoming the first woman to attempt and land a triple-twisting, double-back manoeuvre during her floor routine.
Biles was, however, not quite content even with that unprecedented move.
"It was a lot better. It wasn't as good as in some of the trainings," Biles told reporters.
I'm just happy that I landed it because I feel like after night one my confidence got shot down. So I was really worried about it going into today, that's all I could worry about. So I was really happy.
Four-time Olympic gold medallist Biles, who will head to the 2020 Tokyo Olympics as the prohibitive favourite, finished with a total score of 118.500 to beat Sunisa Lee (113.550) and Grace McCallum (111.850).
"I feel like each (title) gets better and better," she said. "Because it's like the fifth and, then, the sixth, so it just keeps getting more exciting."