No.1 seed Ruud defeated Federico Coria 6-3 6-4 in the last eight on Saturday (AEDT) to set up a semi-final against another Argentinian in Federico Delbonis.
Delbonis defeated Fabio Fognini 6-4 6-4 to make his first semi since losing to Pablo Carreno Busta in Hamburg last July.
The 31-year-old has not played an ATP Tour final since winning in Marrakech in 2016.
There will be a home representative remaining in the other half of the draw, too, although his identity is still to be confirmed.
Diego Schwartzman and Francisco Cerundolo were tied in the third set when rain saw play suspended for the day.
The winner of that match will play Fognini's compatriot Lorenzo Sonego, a straight-sets victor against Fernando Verdasco.
At the Dallas Open, there is an all American cast for the semi-finals, but No.1 seed Taylor Fritz is not among those in the last four.
He lost a third-set tie-break to countryman Marcos Giron, whose reward is a match against Jenson Brooksby. Brooksby beat Australia's Jordan Thompson 6-3 6-0.
John Isner breezed through in straight sets and will face Reilly Opelka.