Williams was a late support slot addition for Mars' two-night stint at the fabled music mecca and he brought leading lady and new mum Gwen Stefani onstage for a rendition of their tune Hollaback Girl.
The couple has been teaming up a lot lately - Stefani joined Williams for his headlining set at the Coachella festival in California in April (14) and the two pals will join forces again as first-time judges on the next season of American Tv talent show The Voice.
Mars, who was making his debut at the Bowl, stunned fans with an energetic hits-laced set and covers of Led Zeppelin's Whole Lotta Love and Montell Jordan's This is How We Do It, among others.
But he left the best until last when he performed an encore featuring the hits Locked Out of Heaven and Gorilla and brought Pharrell back out onstage for a rendition of N.E.R.D.'s 2002 hit Rockstar.
Mars and Williams return to the Hollywood Bowl for a second show on Sunday night (01Jun14).
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