The Winter Olympic gold medalist cruised to victory in splendid conditions at Cardrona's Snow Farm.
The top-seeded female athlete going into the women's prologue, she set the pace for the duration of yesterday's racing to back up her win at PyeongChang last year.
''It was super fun out there,'' she said. ''It was really awesome, a really cool opportunity and so much fun to race with these girls.''
It was a similar story in the men's event as Japan's Hiroyuki Miyazawa, also the top-seeded competitor going into the prologue, remained in front through the heats, semifinals and finals.
''I was so happy to win and concentrate on my tactics and technique,'' Miyazawa said.
''I like this course - not too tough, not too easy, a good course.''
He topped a Japanese clean sweep on the podium alongside countrymen Hikaru Fujinoki and Takanori Ebina in second and third respectively.
Australian Katerina Paul (23) was second behind Diggins.
She was overjoyed with the result as she even exceeded her own expectations.
''It feels amazing. I was quite surprised.
''Going up the hill I had this confidence today that I've never really had much of before.''
Yukari Tanaka finished third
The cross-country 5-10km freestyle interval starts at Snow Farm today.
