American Williams, chasing a third consecutive triumph at the premier clay event in the Spanish capital, dispatched the 17-year-old Swiss, ranked 98th, 6-2 6-1 to set up a meeting with unseeded Chinese Peng Shuai.
Bencic, the youngest player in the top 100, broke the Williams serve once in the opening set but crumbled thereafter and lost her own five times.
Williams was looking fresh after a month off following her second-round defeat in Charleston and sealed victory with her 11th ace of the match for her 13th consecutive win in the Spanish capital.
"I was playing such a young player and she had nothing to lose and she has such a bright future so it wasn’t very easy going out there," Williams said in an interview made available by the WTA.
"But it feels good to get a win under my belt on clay," added the 32-year-old, a 17-times grand slam singles champion.
"I think I played well, I went for a lot of winners and they actually went in so I was like wow.
"I love the crowd here, I love the Spanish. They just have so much life."
Williams was joined in the second round by Czech fifth seed Petra Kvitova, who had a tough test against unseeded Romanian Sorana Cirstea before coming through 6-1 5-7 7-6.
Seventh-seeded German Angelique Kerber was trailing French qualifier Caroline Garcia 6-3 2-0 when she had to retire because of a back injury.
"I've been feeling my back for a few days now and felt it again in the first set and it was getting worse as the match went on," Kerber said.
Maria Sharapova of Russia, the eighth seed, and 11th-seeded Serb Ana Ivanovic also went through, seeing off Czech Klara Koukalova and American Madison Keys respectively.
(Reporting by Iain Rogers, editing by Ed Osmond)
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