Early tries from Danny Barrett and Perry Baker, however, put the Americans in front before Kenya responded with a brilliant solo try from Collins Injera and a breakaway by Billy Odhiambo to make it 14-14 at halftime.
Martin Iosefo and Zack Test both crossed the line in the second half to seal a deserved win for the home team, who will next face seasoned sevens campaigners Fiji.
"I thought our attitude and execution were solid tonight, the best we have had here this week," U.S. head coach Mike Friday said. "We knew how well they (Kenya) were playing, so this was a good step for us."
Kenya's victory over the All Blacks meant they joined South Africa and Australia as the only teams to remain unbeaten after their first three games in pool action.
"Getting one over the big boys is so exciting for us," said Kenya's Collins Injera.
Fiji, who are gunning this season for their third World Series title, combined power with speed to demolish Japan 43-7 after scoring four tries in the first half, Jasa Veremalua crossing the line twice.
Three-time USA Sevens champions South Africa had too much speed and tactical superiority for New Zealand in a clash of rugby heavyweights as they triumphed 14-7.
Seabelo Senatla was lightning-fast on the breakaway and scored twice early on to give South Africa a 14-0 halftime lead, and New Zealand had Kurt Baker sin-binned for a late block on Rosko Specman.
Lewis Ormond broke through a tackle to score right under the posts early in the second half for New Zealand but the Springboks then dominated territorially to secure victory.
South Africa will next take on Australia, who were comfortable 26-12 winners in their quarter-final against Argentina.
Ed Jenkins, Allan Fa'alava'au and Sam Myers each crossed the line to put Australia ahead 19-0 at halftime, and they maintained control to book their place in the last four.
The Las Vegas event is the fifth stop on the 10-tournament World Series circuit. The teams then head to Vancouver, Hong Kong, Singapore, Paris and London over the coming months.
The World Series is providing a showcase for sevens as it prepares for its debut at the Olympics in Rio de Janeiro in August.
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Nick Mulvenney)
