Sympathovagal imbalance is known to precede the onset of atrial fibrillation (AF) and has been analyzed extensively based on heart rate variability (HRV). However, the relationship between sympathetic and vagal effects before AF onset and their influence on various HRV features have not been fully elucidated. QT interval variability (QTV) reflects sympathetic activity and may therefore provide further insights into this relationship. Using the time delay stability (TDS) method, we investigated temporal changes in coupling behavior before AF onset between 20 vagal or sympathovagal-associated HRV and QTV features. We applied the TDS method to 26 electrocardiograms from the MIT-BIH AF database with at least one hour of sinus rhythm preceding AF onset. Sinus rhythm segments were split into 5-minute windows with 50 % overlap. Logistic regression analysis revealed significantly (p<0.01) increased coupling between QTV and vagal HRV features from 20 to 15 minutes before AF onset. We found similar behavior between QTV and sympathovagal HRV features. This indicates sympathetic predominance increasing until 15 minutes before the onset of AF and decreasing towards vagal predominance right before AF onset. Our results provide new insights into temporal changes of sympathovagal imbalance preceding AF onset and may help predict AF in clinical applications.