The attacker was being plagued by muscular problems in the past few days, but has been deemed match fit again ahead of Tuesday's game with Dortmund.
The Portugal international netted his side's only goal in a 4-1 first leg defeat last week, but then missed this weekend's 2-1 win over Atletico Madrid due to a leg injury.
Nevertheless, Ronaldo is not in danger of missing Tuesday's return at the Santiago Bernabeu as he has resumed regular training action again after a brief layoff.
The prolific attacker joined his teammates for the first time for Monday's early session and participated without any noteworthy problems.
Ronaldo has been in superb form for Madrid this term, netting 51 goals in 50 appearances in all competitions, and Madrid will be hopeful he can add to his tally on Tuesday as it aims for a comeback win over Dortmund in order to make it to the Champions League final.
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