Motorhead's 'Ace of Spades' takes on the Kinks' 'You Really Got Me' in the semifinals of Ultimate Classic Rock's March Riff Madness.

Three weeks ago, we started with 32 of rock’s most iconic riffs. Now, that field has been whittled down to four. We’ll keep going until we theoretically arrive at the most awesome riff in all of rock history.

Motorhead reached the final four by defeating Black Sabbath's 'Iron Man,' Deep Purple's 'Smoke on the Water' and Ozzy Osbourne's 'Crazy Train' in succession. The Kinks arrived here by trampling AC/DC's 'Back in Black,' Led Zeppelin's 'Whole Lotta Love' and Rush's 'Limelight.'

So who moves onto the finals? It's up to you! Everyone can vote once per hour until the semifinals of March Riff Madness closes on Sunday, March 30 at 11:59PM ET. The ultimate March Riff Madness winner will be revealed on Tuesday, April 7.

Watch Motorhead Perform 'Ace of Spades'

Watch the Kinks Perform 'You Really Got Me'