That's a difficult question.
I think both fell in love with each other almost at the same time but Adi realized it first.
When Adi saw Zoya for the first time, he got attracted to her but it was not love. During the end of the jogi mahi song and the garland scene, both got attracted to each other.
After Adi saved Zoya when she was stuck on the terrace, he realized for the first time that both their pains are same. What he did that day to her was his humanity and empathy.
In the riots scene, Zoya realized Adi was not a bad person and she was mesmerized by his good nature just like he was mesmerized by her beauty in their first meeting.
After Zoya's suicide attempt, Adi wanted both of them to forget each other's past and he realized only both of them can help each other in that so he wanted to be Zoya's friend.
Love between them blossomed during the jungle track. Both fell in love with each other during that track. What they felt for each other was more than attraction but due to those circumstances and their past, they didn't realize it. After Rajveer track ended, both again were in loggerheads during the paak rishta track.
After that, leap happened. During the 6 months gap though both stayed away from each other, their love grew. And then after the leap, Adi realized his love first.