In the process, Guardians of the Galaxy passed Transformers: Age of Extinction to become the highest-grossing movie of the Summer at the domestic box office.
Among the newcomers, If I Stay had a solid debut, while Sin City: A Dame to Kill For imploded.
The Top 12 earned $102.5 million this weekend. That's up 14 percent from the same frame last year.
Guardians of the Galaxy eased 32 percent to $17.2 million. It has now earned over $251 million, and by Friday it will pass fellow Marvel movie Captain America: The Winter Soldier ($259.8 million) to become the top grosser of the year. It also now seems likely that Guardians will end its run with over $300 million.
In second place, Teenage Mutant Ninja Turtles fell 41 percent to $16.7 million. Through 17 days, the movie has earned $145.5 million, and is on track to finish with at least $175 million.
Playing at 2,907 locations, If I Stay took third place with $15.7 million. That's a step up from Charlie St. Cloud ($12.4 million) and last weekend's The Giver ($12.3 million). It's also worth noting that it opened on par with One Direction: This is Us ($15.8 million), which was released around the same time last year and was also targeted toward younger women.
As expected, the audience did wind up skewing young (61 percent under 25) and female (77 percent). While If I Stay did receive a solid "A-" CinemaScore, it's the kind of movie that tends to fall off quickly after opening weekend; a final total around $40 million seems likely.
Let's Be Cops was crushed by critics, but moviegoers seem to be in to it: the comedy eased 39 percent to $10.8 million this weekend. To date, it has earned $45.1 million.
High school football drama When the Game Stands Tall rounded out the Top Five with $8.4 million. That's a bit lower than recent sports movies Million Dollar Arm and Draft Day, but is still essentially in the same (low) range.
It received an "A-" CinemaScore, and should play to family audiences for the next few weeks. Look for a final total around $25 million.
Sin City: A Dame to Kill For opened in eighth place with just $6.32 million. That's down a stunning 78 percent from the original Sin City's debut. Excluding his first movie, El Mariachi, this is the third-lowest debut of director Robert Rodriguez's career (Machete Kills and Shorts were worse).
During the nine-year gap between the first and second Sin City, a few key things happened. First, the comic book aesthetic that made Sin City so unique has been cribbed by many other movies; ironically, some of the most obvious imitators are fellow Frank Miller projects 300 and The Spirit. What was once a fresh approach now has a "been there, done that" feel to it.
Adding to this is the fact that director Robert Rodriguez's brand has taken a bit of a hit in recent years. Specifically, Rodriguez has had a tough time selling sequels: Spy Kids: All the Time in the World was off 66 percent from its predecessor, while Machete Kills was down 70 percent.
The movie also had a fairly light marketing effort: anecdotally, many people seemed surprised to learn that the movie was in theaters this weekend.
Finally, it's worth considering whether or not the first Sin City was actually as well-liked as it appears to be. The movie received good reviews and has a spot on IMDb's Top 250 list, but that sampling could be inconsistent with more general views on it.
Looking ahead, Sin City: A Dame to Kill For seems poised to fall off quickly. It's not going to close anywhere near $20 million, and could even fall short of $15 million.
Lucy led the overseas box office this weekend with $34 million. The Luc Besson thriller opened to a massive $6.3 million in Taiwan, where a significant portion of the movie is set. It also had strong debuts in the U.K. ($5.2 million) and Spain ($3.5 million). So far, Lucy has earned $103 million overseas, and still has many markets left to open; it expands in to Brazil, Japan and Mexico next weekend.
Without any major new markets, Guardians of the Galaxy fell 49 percent to $20.7 million. It has grossed $237.6 million so far, and still has Germany, Italy, Japan and China on the way.
How to Train Your Dragon 2 added $18.2 million this weekend, which pushed its total past $400 million. China is now its biggest market with $48.2 million.
Teenage Mutant Ninja Turtles took in $15.5 million, bringing its total to $93.2 million. It has yet to open in most major markets.
Discuss this story with fellow Box Office Mojo fans on Facebook. On Twitter, follow us at @boxofficemojo, and follow author Ray Subers at @raysubers.
This Weekend's Forecast:
• Long Delay Will Slay 'Sin City' Sequel This Weekend
This Weekend in Past Years:
• 2013 - 'Butler' Repeats, Newcomers All Open Below $10 Million
• 2012 - 'Expendables 2' Repeats on Awful Late-August Weekend
• 2011 - No Downgrade for Hurricane 'Help'• 2010 - 'Expendables' Battle On, 'Vampires,' 'Piranha' Settle for Scraps
• 2009 - 'Inglourious Basterds' Scalps the Box Office
• 2007 - 'Superbad' Stays on Top
• 2006 - 'Invincible' Tackles Top Spot
• 2005 - 'Virgin,' 'Grimm' Top Glumm Weekend
• Weekend Box Office Results
• All-Time Domestic