Complex sentence: The highlight of Davis's athletic career was his participation in the 2004 Summer Olympic Games in Athens, Greece, where he qualified in the 400m and 4x400 relay for his mother's country of Great Britain.
Split into simpler sentences: The highlight of Davis's athletic career was his participation in the 2004 Summer Olympic Games in Athens, Greece. When competing in the Olympics, he qualified in the 400m and 4x400 relay for his mother's country of Great Britain.