Taylor Swift's special guest at the Oct. 1 stop of her 'Speak Now' world tour in Atlanta was none other than R&B crooner Usher. This tour is really shaping up to be landmark in terms of the songs that Swift covers and the guests she brings on stage with her when she rolls through town!

When Usher -- dressed in black jeans and a gold topper against Swift's familiar purple frock -- hopped on stage, it was Swift combining the two key elements of her tour into one: a special guest and a unique cover song.

Judging from the screaming, almost rabid fans in this clip, Swift and Usher brought the house down and shook its very foundation with their rendition of his 2004 No. 1 song 'Yeah!' Swift really is, to borrow a title of one of her albums, "fearless" when she gets onto the stage. She is not afraid to attempt straight ahead pop, rock, emo, hip-hop and R&B when she does her covers and sings with her guests.

Usher and Swift got down and grooved together, which could set the table for a future collaboration between the country pop cutie and the smooth-like-butter R&B superstar. Just sayin'. This is how these seeds get sown.

Watch Taylor Swift an Usher Perform 'Yeah!'