Ronaldo, who met Liverpool-born Clayton through American sports nutrition company Herbalife, entrusts the 37-year-old with improving his speed.
He shared a picture of the pair running on his Instagram page last month with the caption: “Working on speed skills with athlete and fitness expert Samantha Clayton.”
Clayton, formerly known as Samantha Davies, competed for Great Britain at the 2000 Sydney Olympics in the 200m and 4x100m relay track events.
She currently lives in California with husband Royce Clayton, the former San Francisco Giants baseball player.
Ronaldo, meanwhile, was absent on Wednesday night as Madrid smashed four past Leganes to stay at the top of La Liga.
Los Merengues boss Zinedine Zidane rested Ronaldo, Gareth Bale and Toni Kroos against the Spanish strugglers – but that didn’t matter as James Rodriguez opened the scoring before Alvaro Morata netted a hat-trick.
Ronaldo, who has scored 35 goals for club and country this season, is expected to return to Madrid’s line-up for Saturday’s derby with Atletico Madrid.