Whether the defendant's counsel had petitioned for setting aside pre-decree?, then contest his petition with valid grounds on your side, only the court will decide the merits and pass orders; If you feel that the defendant is not having and valid evidence or ground to contest the pre-decree, why do you worry about the out come of the petition for setting aside the pre-decree filed by the defendant.
After the ex-parte decree is passed by the competent Court, the defendant need to file Application for setting aside the ex-parte decree. He must satisfy the Court, why the decree should be set-aside. In your case if such application is moved then you must contest the same and get it dismissed. Then he has the remedy by way of appeal. Since it is a specific performance suit he has to show is ready and willingness in order to succeed.