Who says the first week is better than the second when it comes to multi-week music festivals? Foo Fighters put that assumption to the test when they took the stage in Austin Friday night for the second week of the Austin City Limits Music Festival and they had a few surprises in store.

Yes, as you would guess, the show did have some special guests, but the bigger surprise came during the introduction of one of the guests. "You know what I did last night? I recorded a beautiful song in your f--king city," revealed vocalist Dave Grohl. "And I'm just gonna say it now … We recorded five f--king new songs in your beautiful city last night. We're gonna give it to you, but not tonight."

He went on to add, "Just by chance I ran into an old friend I hadn't seen in about 16 years right as I was recording a vocal. I'm like, 'Hey man,' and this motherf--ker starts singing the most beautiful harmony to what I was singing. And I was like, 'Get your ass into the vocal booth right now.' And then he said, 'Hey, if you want I'll come up and sing a song with you tonight.'" As can be seen in the video above, Grohl's guest was Ben Kweller and they proceeded rock the "sweet, tug-your-heartstrings ballad" "Big Me."

The other major guest appearance of the evening came from the Foo Fighters recent support act and Austin native Gary Clark Jr., who helped the band reprise their Sonic Highways song "What Did I Do? / God as My Witness" with a blistering guitar solo. Check out video of that performance below.

While the guest performances are cool, the bigger take away from evening was the revelation that Foo Fighters are back working on new music. No timetable has been set for the release of a new album and it is not known if the songs are part of a proposed Season 2 of the Sonic Highways documentary series. Stay tuned to see how it plays out.

Watch Foo Fighters Perform "What Did I Do? / God as My Witness" With Gary Clark Jr. at the Austin City Limits Festival

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