The American next meets second-seeded Serb Jelena Jankovic, who cruised to a 6-2 6-0 victory over Slovenian Katarina Srebotnik.
Fourth seed Dinara Safina will play fellow Russian and ninth seed Vera Zvonareva in the other semi-final.
"We have both played well to get to the semis so I'm looking forward to playing the same kind of tennis, aggressive but with patience," Williams told reporters.
Medina Garrigues had high hopes of staging a fightback when she led 3-0 in the second set but the world number 29 had other ideas.
"I just felt like 'I'm not losing this set'
The two women have not met since Jankovic's upset win over Williams in the third round at Wimbledon last year.
"I didn't expect to win so easy as I did today because we have had some tough matches," said world number nine Jankovic. "I stayed focused and I did well."
Zvonareva battled a sore wrist in her 6-1 7-5 win over unseeded Dutch player Michaella Krajicek, while world number 12 Safina emerged from the late match, when 10th seeded Tatiana Golovin of France retired with an ankle injury after losing the first set 6-3.
"I'll get treatment (for the wrist)," Zvonareva said. "I'll do everything I can to recover and then we'll see what's going to happen."