Hot 100 March Madness Finals: Vote Now!
Hot 100 March Madness Finals: Vote Now!

And then there were two: after four rounds of Billboard.com's Hot 100 March Madness, the fans have helped two pop superstars conquer the bracket and reach the Finals. Britney Spears' "Hold It Against Me" and Katy Perry's "Teenage Dream" are the No. 1 Hot 100 singles that have proven unstoppable thus far in the competition, and now, they must face off against each other in one final round of voting!

To help determine the winner of Hot 100 March Madness, check out the updated bracket of songs, and vote for your favorite No. 1 single below.

The original bracket was comprised of the 32 most recent songs to top the Hot 100 chart. The match-ups within the bracket were determined by the dates in which each song topped the Hot 100: the most recent No. 1, fun.'s "We Are Young," is matched up against the 32nd most recent No. 1, Rihanna's "Rude Boy," and so on. Click here to see the original bracket, and check out the Sweet 16 and Elite 8 here.

On Friday, Mar. 30, a Hot 100 March Madness champion -- the No. 1 single that you think is the best smash hit of the past two years -- will be crowned. Will it be "Hold It Against Me" or "Teenage Dream"? It's up for you to decide!

Looking back at the Final Four, over 80,000 votes helped push Britney and Katy past their equally formidable competitors. 53% of fans voted for "Hold It Against Me" over Adele's "Set Fire To The Rain," while 62% of fans picked "Teenage Dream" over Eminem's "Love The Way You Lie," featuring Rihanna. For those keeping score at home, Britney's latest No. 1 hit -- the first single from her seventh album, "Femme Fatale" -- trumped Wiz Khalifa's "Black and Yellow," Rihanna's "Rude Boy," Bruno Mars' "Just The Way You Are" and "Set Fire To The Rain" to make it to the Finals, while the second single from Perry's blockbuster "Teenage Dream" disc has defeated Adele's "Someone Like You," Ke$ha's "We R Who We R," B.o.B's "Nothin' On You" and "Love The Way You Lie" thus far.

Will Britney or Katy earn the very first Hot 100 March Madness crown? You only have until Friday to weigh in! Vote now for your favorite track below, and be sure to discuss the competition in the comments section!