A sidewall of biological tissue which surrounds and defines an opening in the tissue is sealed or fused to occlude the opening by compressing apposite sidewall portions and applying sufficient energy to cause the fibers of the compressed apposed sidewall portions to intertwine and fuse with one another to form a permanent seal. The energy application is controlled by detecting a precursor fusion condition while applying the energy and before sufficient energy has been applied to achieve an adequate seal. The application of energy is terminated in a time-delayed relationship to the detection of the precursor fusion condition such that sufficient energy has been applied to achieve an effective seal. The precursor fusion condition is detected upon the peak RF current delivered to the tissue remaining below a threshold value for a threshold time.