He was level with the main challenger, Poland's Natalia Kaczmarek, with 120 meters to go but then extended the distance to win by half a second.
Recently, Bol went under 50 seconds in the 400m flat for the first time by running 49.75 and placing first in the European statistics.
Tomorrow she enters the special distance 400 m hurdles by running the semi-finals.
As a seeded runner, she had to sit through the test that was this morning.