She ran a comfortable 53.48 on Monday evening in Rhine-Ruhr on the first day of track and field in Germany, finishing first in her heat to qualify for the semifinals along with teammate Precious Molepo.
Speaking to SABC Sport shortly after her race, Viljoen admitted she was still cautious and not putting pressure on herself to retain the gold medal.
"I'm just grateful to be able to run. I think we take so many opportunities like this one for granted, when you're injured, you'll give anything just to run.
"My mental capacity and ability to overcome has improved so much after my injury, and I just have a brand new outlook on running," said the talented Viljoen.
The 24-year-old Tuks student added that she did not take representing her country on the international stage for granted.
Molepo also reacted to being in the semis on Tuesday night, revealing she was targeting a bronze medal in the 400m.
"I just try to focus on my own lane, and I'll be going all out for a place in the final, and I hope to return home with a bronze medal."