The Team Ascendis Health rider won in 5:46:26 to finish the 121km race between Outdshoorn and Great Brak River in 12th place overall.
She was followed home by Swedish champion Jennie Stenerhag in 5:49:45.
After taking the Queen of the Mountains hotspot midway through the race, Swiss defending champion Ariane Kleinhans was hampered by a flat and eventually finished third in 6:09:07.
Stenerhag was leading at the second water point, but De Groot closed the gap shortly afterwards.
The men’s race was won by Germany’s Karl Platt (Team Bulls).
Visit Attakwas Extreme MTB Challenge for more news, photos and results.