The eighth round of Hardly Strictly Bluegrass brought together regulars on the bluegrass circuit and artists who are not bluegrass at all. The third day showcased this diversity, with Elvis Costello's High Whines & Spirits strutting their stuff on the same day as bluegrass pioneer Earl Scruggs. The rest of the festival placed equally formidable non-bluegrass talents on stage, from rocker Nick Lowe to indie-folk heroes Bonnie 'Prince' Billy and Iron and Wine. One of the biggest acts, however, was the emerging twosome consisting of a legendary frontman and a sweet soprano.

Upstaging the powerful MC Hammer, Robert Plant and Alison Krauss delivered a sublime performance of their collaborative blues at the Banjo Stage. Opening with the suga momma song "Rich Woman," the duo put a hush over Speedway Meadows in Golden Gate Park. The lead track from last year's Raising Sand, it filled the venue with a sly fingers-crossed duet.

Helping Plant and Krauss was a back-up band led by veteran producer T-Bone Burnett. The band was as prominent as the two singers acoustically, especially the stand-up bassist who laid down the foundation for blues. Yet neither Plant nor Krauss were afraid to show off their instrumental skills. At one point Plant graced the stage with some three-finger banjo picking, perhaps in tribute to Earl Scruggs' upcoming appearance. Alison Krauss offered a sweet helping of fiddle in a few solo songs, the best being "Down to the River to Pray" from Oh Brother, Where Art Thou?.

Plant got a chance to play some solo work too, which pleased many a Zeppelin fan in the audience. The acoustic version of "The Battle of Evermore" stood out from the rest. The bluegrassed re-interpretation of the song didn't have Jimmy Page's guitar but felt just as epic.

The solo work, while strong, never came close to the power and chemistry that Plant and Krauss offered together. Songs like "Gone Gone Gone" and "Sister Rosetta Goes Before Us" displayed Krauss' vocal range and strength. Yet it is the tenderness that Plant reveals in these duets that is the biggest treat. The best example of this was the melancholic pleas of "Please Read the Letter." Nearing the end of the set, Plant and Krauss sang the song with a perfect balance of heartbrokenness and hope. It probably even made Hammer cry.

Rajesh Srinivasan and Stefanie Lee of The Daily Californian contributed to this review.