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(Yahoo!) - As if Jay-Z's epic Yahoo! Wireless Festival performance could have been any better, he brought out a special guest for his final song: Justin Timberlake.

The duo powered through a propulsive debut rendition of Holy Grail that had the entire crowd leaping into the air.

Justin appeared to raucous applause under a bright red baseball cap before his liquid vocals began rippling out over the crowd. When the beat proper dropped in we thought the ground of the Olympic Park would begin to crack from the madness that kicked off.

It was the second special guest appearance the duo shared at the festival. During Justin's Friday night headline performance (which also featured the debut of a brand-new song titled "Take Back the Night") Jay-Z came out for a run through of "Suit & Tie."

Jay-Z had been watching Justin's set from side of stage with JT's wife Jessica Biel, Frank Ocean, Snoop Lion, Rita Ora, and more before joining the man himself.

With two sneak previews down, we could not be more excited for the real deal tomorrow when Jay-Z and Justin join forces as the Legends of the Summer for the Sunday headline show.


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