The top seed, who ate dog food before her win over fellow American Christina McHale in the last round, beat Russian Kuznetsova 6-2 6-0 in 51 minutes.
Williams will now play Romania's Irina-Camelia Begu after she beat Japan's Misaki Doi 6-2-7-6.
In the other semi America's Madison Keys faces Spain's Garbine Muguruza.
Keys, 21, despatched Barbora Strycova of the Czech Republic 6-4 4-6 6-3, while 2015 Wimbledon finalist Muguruza beat Switzerland's Timea Bacsinszky 7-5 6-2.
Williams, who admitted being sick after eating dog food ordered off a menu for her Yorkshire Terrier, is aiming for a fourth Italian Open title.
World number one Serena Williams waltzed through to the Italian Open semi-finals by defeating ninth seed Svetlana Kuznetsova in straight sets.