The 35-year-old Juventus forward last weekend challenged his Instagram followers to a "core crusher" test, with the former Real Madrid and United star hitting an impressive 142 reps in just 45 seconds.
Alongside a number of sporting figures, Fernandes - who has starred for Ronaldo's former club since making a January switch to Old Trafford - tried the challenge out for himself.
Upon completing the workout, he wrote on Instagram: "#NikeLivingRoomCup - Week One. 117 Abs in 45 seconds. Not bad for a Sundayâ€¦"
However, Ronaldo - clearly less than impressed - replied on his Instagram story saying: "Train your arms instead!"
Fernandes may not have lived up to Ronaldo's impeccable standards, but teenage sensation Erling Haaland - who on Tuesday revealed Ole Gunnar Solskjaer as the key behind his goal-scoring prowess - fared better.
The Norwegian striker managed 124 crunches in the allotted 45 seconds, with Ronaldo showing his approval on his Instagram story.
The five-time Balon d'Or winner may be leading the way in the football war, but South African runner Caster Semenya smashed the record with 176 reps.
She subsequently joked that Ronaldo had "tried his best".
Who will come out on top remains to be seen. But for Bruno Fernandes, more work is needed!