Double Olympic champion Mo Farah was beaten into second by American Garrett Heath in the men's race at the Great Edinburgh Cross Country.
Farah, 32, lost out in a sprint finish as Heath won in 25 minutes 29 seconds.
"It is disappointing to lose but better to lose now than later in the year. I couldn't really sprint - it was very slippery," Farah told BBC Sport.
Fellow Briton Kate Avery, 24, recorded her first senior international victory in the women's race at Holyrood Park.
Avery took the lead in the early stages and finished the 6km course in 21:05 to win by four seconds.
Ireland's Fionnuala McCormack, representing Europe, was second, with GB's Gemma Steel third in 21:31.
Great Britain won the team event with Europe second and USA third.
Scotland A, anchored by Laura Muir, won the 4x1km mixed invitational relay with Britain second and Ireland third.
Britain's Alex Yee, 17, won the junior men's race with compatriot Bobby Clay, 18, victorious in the women's junior event.
Farah struggles in rainy conditions
Farah, competing in the event for the first time since winning in 2011, began his Rio Olympics preparations with a rare defeat over the 8km course.
The five-time world outdoor champion was at the back of the field in the early stages and, although he gradually moved through the pack, he appeared to struggle on fields made muddy by heavy rain.
Britain's Ross Millington and Callum Hawkins made the early running but Heath, 30, produced a decisive move on the final 1.5km lap to beat Farah by two seconds.
American Scott Fabul was third in 25:38 with Hawkins (25:41) fourth and Millington (25:43) fifth.
Emotional win for 'bridesmaid' Avery
Avery, a silver medallist in the European Cross Country Championships in 2014 and 2015, was thrilled with her victory, saying that previously she was "always a bridesmaid".
The win came after the recent death of her father and she told BBC Sport: "He's still my biggest supporter. I am sure he was pushing me round out there - that's why I ran so well.
"I knew I was in shape but you never know how your body will react. It was so cold.
"Everything is geared towards Rio, which is my main aim this year."