Ronaldo had challenged fans to take part in a 'core crusher' over the weekend, with the 35-year-old amassing 142 reps in just 45 seconds.
Manchester United newboy Fernandes managed 117 reps in his attempt, writing on Instagram: '#NikeLivingRoomCup - Week One. 117 Abs in 45 seconds. Not bad for a Sunday…'
But Ronaldo took a friendly shot at Fernandes, replying: 'train your arms instead!'
Fellow United player Diogo Dalot also took part in the challenge, finishing with 105 reps.