She set up break point by penalizing Williams for coming forward, then converted when Williams pushed a sitting, open court backhand long.
Through four games, Williams, whose groundstrokes had trouble clearing the net, hit zero winners to nine unforced errors, a stark contrast to Haleps six winners to zero unforced errors.
Williams will look to take positives out of the final third of that set, where she struck eight winners and won eight consecutive points on serve to settle in.
Playing in her third major final since becoming a mother to daughter Alexis Olympia Ohanian Jr, Williams hopes the third time is the charm as she looks to tie Margaret Court for an all-time record 24 major titles.
Williams is 7-3 in title matches at the All England Club, 23-8 overall in Grand Slam finals, and already holds the distinction as the oldest woman in the Open era to win a major, with her 2017 Australian Open victory over sister Venus.