Frankly, I was confused walking into the Potawatomi Bingo Casino Stage for Fitz and the Tantrums late Friday evening. Admittedly, I'm not the most seasoned veteran of Summerfest, but I was always under the assumption that the humble stage was for small acts that drew a more laid-back and sit-down kind of crowd. I wasn't quite sure the petite venue was ready for the world-touring indie soul group.

As it turns out, the Potawatomi Stage was perfectly prepared for the band and the packed crowd. It was I who was ill prepared for Fitz and the Tantrums' infectiously energetic and sultry set that somehow managed to make the balmy night even steamier.

The modern soul group got its start in Los Angeles in 2008, gathering up gigs and picking up fans before releasing their first and currently only album, "Pickin' Up the Pieces." The 2010 record attracted some impressive attention for Fitz and the Tantrums, including from VH1, who gave the musicians their "You Oughta Know" stamp (a title previously given to artists like Regina Spektor, Adele and Mumford & Sons) and a gig as the house band for the channel's airing of the 2012 Critics' Choice Movie Awards.

They also caught the ear of Maroon 5 front man Adam Levine, who made the jazzy up-and-comers their opening band for several shows, including last year's Summerfest appearance at the Miller Lite Oasis.

One year later, Fitz and the Tantrums are no longer the opener but the headliner and deservedly so. They set the tone early with their opening numbers, the groovy "Don't Gotta Work It Out" and "Winds of Change," which thoroughly showcased the band's sassy attitude and vivacious on-stage energy.

Considering the humid temperature, it'd be easy for the group to lose their spunky spirit as the show went on. Somehow, though, Fitz and the Tantrums became even more lively and entertaining to watch. During the snappy "L.O.V.," lead singer Michael Fitzpatrick hopped across the stage, bounding from the top of the speakers on the left of the stage to Jeremy Ruzuma's keyboard on the opposite side in between leading the crowd in an organized clap.

Fitzpatrick wasn't alone either. Vocalist Noelle Scaggs was constantly dancing with a fascinating rubber-like looseness and banging her tambourine so much that after "Dear Mr. President," her instrument was broken (luckily, she had a back-up on standby).

Put Fitzpatrick and Scaggs on stage together, singing about love gone wrong, and the two performers ignited the stage with a sensual heat that made Milwaukee's recent heat wave seem lukewarm by comparison. During "News 4 U," it seemed like Fitzpatrick could barely take his eyes off of the dancing Scaggs. Their wild chemistry was equally palpable during "Breakin' the Chains of Love" as the two sang at one another barely inches apart.

Their sexy charisma perfectly matched the music's dirty soulful feel. "Tighter," Fitzpatrick's favorite song off of their album, was a sensual slow jam that got into the audience's skin, and their encore cover of "Sweet Dreams (Are Made of This)" took the Eurythmics' dark and brooding original and transformed it into a wonderfully jazzy number.

Of course, it helps when the musical parts are well played and properly balanced. Each part of Fitz and the Tantrums' six-part band could be heard and was given a moment to shine. James King's sax got to steal the spotlight for a sweet solo in "Breakin' the Chains of Love," while "Pickin' Up the Pieces" and "6 AM" (a new song off their upcoming album) showcased Fitzpatrick and Scaggs' ear-pleasing harmonies.

Perhaps the show's musical highlight – no easy task to choose just one – was the climactic and chaotic breakdown at the end of "News 4 U." Even Fitzpatrick, who was dancing and swinging his hair around throughout the concert, could barely keep up the energy throughout all of Wicks' raucous solo.

Fitz and the Tantrums' topped off the 70-minute set with an encore performance of their hit "Moneygrabber." Like many of their songs, it's filled with bitter lyrics about a love reaching its end. The words certainly don't speak to the audience's infatuation with the band, which after their contagious performance, was in full bloom.