CHICAGO -- After spending a sun-soaked day flirting with EDM, hip-hop and other ephemera, Lollapalooza got back to it's roots on Friday (August 3) night, rolling out a pair of hard-riffing rock headliners ... with bizarrely similar names.

The Black Keys and Black Sabbath duked it out on opposite ends of Grant Park, each delivering sweaty, workmanlike sets that reminded the thousands who survived the day's brutal heat (and continuous wub-wub-wub backbeat) that Lolla is, first and foremost, a rock fest. Even if neither of said rock acts really felt the need to state that fact.

The Keys -- introduced by Chicago mayor Rahm Emanuel, who sounded like he'd spent the day bumming smokes off concertgoers -- chugged through 90 minutes of diesel-fueled rock, with frontman Dan Auerbach pulling Vulcanized solos off his frets and drummer Patrick Carney bashing hard and true (for those keeping score at home, he lost his glasses four songs into the set). There was nothing flashy about their set, and they kept on-stage banter to a minimum, preferring instead to just keep their heads down and blast.

And they accomplished that feat in spades. Songs like "Howlin' For You" and "Next Girl" snarled and slunk, "Gold on the Ceiling" was a supercharged stomper and "I Got Mine" simmered for what seemed like an eternity before finally combusting. What the dynamic duo (plus backing band) lacked in showmanship, they more than made up for in sheer volume, flexing some serious muscle on tunes like "Little Black Submarines," "Your Touch" and "Lonely Boy."

And the crowd loved every minute of it. The Keys were so tuned in -- and so loud -- that they even managed to upstage the fireworks that erupted towards the end of their set (whoops!), diving headlong into "Money Maker" and never once acknowledging the aerial show. Sure, some folks may turned their attention skyward, but the Auerbach and Carney brought their gazes back to the stage ... and did so as effortlessly as they did anything else on Friday.

The same could also be said about reunited hard-rock heroes Black Sabbath, who bruised their way through two house and never really got much flashier than the bedazzled cross on Ozzy's shirt. And to the dedicated throng of metal maniacs who caught their set, that was just fine indeed.

Decked out in his glittery shirt (and matching housecoat) iconic frontman Osbourne was in fine scary grandpa mode. He fully committed to the band's signature throbbing march of witchy doom while flashing his best scary eyes and honking on a harmonica during "The Wizard." And forty years has done nothing to diminish the creepy menace of lyrics like "my name is Lucifer, come take my hand," or of guitarist Tony Iommi's arsenal of doom-laden riffs.

Ozzy admitted that "Snowblind" is now an ode to a former vice (we assume the same goes for the bludgeoning "Sweet Leaf"), which didn't diminish the song's gut-punching power. And "War Pigs?" It went off like an A-bomb that rivaled any of the archival WWII footage unspooling behind the band. Even some of Chicago's finest, keeping a close eye on the bleary mob, couldn't resist tapping their heels a bit to the strains of "Iron Man."

This is a rock fest, after all.

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