I assume that your question is not about the exact usage or origin of the phrase First Among Equals rather the primacy of the bishop of Rome. i.e., The office of the bishop of Rome is considered preeminent.
The Orthodox Churches did not begin to assert this title after the Schism nor did the Catholic Church reject it after it. Both the churches always agreed that the bishop of Rome is first among the Pentarchy (Rome, Constantinople, Alexandria, Antioch and Jerusalem). The problem was on whether this primacy is just a honorary title or does it have functions/privileges attached to it (which is in and of itself a completely different question).
There has been many instances before the Schism which shows that the papal supremacy was an accepted fact.
- "For it is with this Roman church, by reason of its more powerful preeminence that every other church, that is to say all the faithful everywhere, ought to agree, inasmuch as in this church the apostolic tradition has been preserved continuously by those who come from everywhere." St. Irenaeus ("Against All Heresies," c. 180 A.D.)
- The first bishop to claim primacy in writing was Pope Stephen I (254-257)
- Where Peter is, There is the Church - St. Ambrose, Archbishop of Milan ("Commentary on Twelve Psalms of David" c. 389 A.D.)
- "'But,' you [Jovinian] will say, 'it was on Peter that the Church was founded' [Matt. 16:18]. Well . . . one among the twelve is chosen to be their head in order to remove any occasion for division." St. Jerome ("Against Jovinian," c. 393 A.D.)
- "Among these [apostles] Peter alone almost everywhere deserved to represent the whole Church." St. Augustine of Hippo ("Sermon 295," c. 411 A.D.)
- Edicts of the Emperor Theodosius II and of Valentinian III proclaimed the Roman bishop as "Rector of the whole Church". The Emperor Justinian, who was living in the East in Constantinople, in the 6th century published a similar decree.
- First Council of Constantinople decreed: "The Bishop of Constantinople, however, shall have the prerogative of honor after the Bishop of Rome because Constantinople is New Rome.
They might have used different words, but neither the Catholic church nor the orthodox church has ever refused the primacy of the Bishop of Rome till today. They always considered him to be First Among Equals. It is whether this primacy has extra function is what is been questioned after the east-west schism.