It was brains over brawn at the box office Friday night, as "Lucy" easily topped "Hercules" for first place.

The Scarlett Johansson thriller -- about a synthetic drug that massively increases brain power -- earned a bigger-than-expected $17.1 million.

"Lucy" cost a reported $40 million to produce, and will likely earn more than that this weekend. Johansson is having a banner year, having already appeared in the smash sequel "Captain America: The Winter Soldier" and the arthouse hit "Under the Skin."

Meanwhile, the Brett Ratner-directed "Hercules," starring Dwayne Johnson in the title role, had a rather weak opening. It muscled up $11 million for second place, and will likely earn around $28 million for the weekend. 

With Marvel's "Guardians of the Galaxy" opening next weekend, "Hercules" will have to look overseas to justify its reported $100 million price tag. It's already playing well in several key markets, including Russia. 

In third place, "Dawn of the Planet of the Apes" picked up another $4.7 million. So far, "Apes" has earned $160.3 million domestically and should have no trouble topping 2011's "Rise of the Planet of the Apes," which ended up with a $176 million domestic haul.

"The Purge Anarchy" continues to scare up big business. The low-budget horror sequel earned another $3.4 million on Friday, bringing its total to $44.7 million.

Rounding out the top five was Disney's struggling "Planes Fire and Rescue," which flew to $2.7 million yesterday. Its North American total stands at a low $28.5 million. Last year's "Planes" soared to a $90 million total, but the sequel won't come anywhere near that amount. 

Look for complete weekend estimates Sunday on HitFix.