Top-seeded Halep recorded her 13th consecutive victory on Madrid's clay courts.
"I was moving great. I felt the game, I like to play on this court," Halep said. "Always when I come back here, I feel a little bit more relaxed, so maybe that's why I can touch my best level every time I play."
Second-seeded Caroline Wozniacki also eased through, dispensing with Daria Gavrilova 6-3, 6-1. The Australian Open champion is playing a week after she withdrew from the Istanbul Cup due to an abdominal injury.
Third-seeded and home favorite Garbine Muguruza also advanced with a 6-4, 6-2 win over Shuai Peng. The Spaniard hit five aces and broke Peng's serve four times.
Azarenka, who had reduced her playing schedule due to a custody dispute with her son's father, said that she planned to play at Rome, the French Open, Mallorca and Wimbledon.
"I'm trying to get a little bit more stability right now,'' the Belarusian said. "As much as I can play, I will.
"I was so happy to be on that plane, you have no idea. You have no idea how happy I was to just have a fresh European air, European food, everything.''
Other winners included Samantha Stosur, Sorana Cirstea and Carla Suarez Navarro.