The 37-year-old American dismantled Halep's game to race through the opening set in 20 minutes.
Romania's Halep, 27, recovered in the second, breaking in what proved to be the final game.
Williams, seeded 16th, saved three break points for 3-3 in the decider, crucially winning the next game to seize control.
The 23-time Grand Slam champion comfortably held her next service game to edge closer, then served out for victory over last year's runner-up in one hour and 47 minutes.
