Keys, the 2014 and 2023 Eastbourne title-winner, has still to drop a set at this year's edition following a 6-3, 6-1 quarter-final win over McCartney Kessler in an all-American clash.
Meanwhile, 2021 Eastbourne champion Ostapenko cruised into the last four with a 6-3, 6-0 defeat of Zeynep Sonmez, despite suffering from heat stroke the previous night.
Keys and Ostapenko are the last two seeds left in the women's draw.
They each have one Grand Slam singles title to their credit, with Latvia's Ostapenko the 2017 French Open champion and Keys triumphing at last year's Australian Open.
Keys will face Petra Marcinko in her semi-final after the Croatian defeated Caty McNally 6-3, 4-6, 6-4 in two hours and 18 minutes.
"I'm very happy with today's performance," said Keys after reaching her fourth semi-final on Eastbourne's grass courts.
Former Wimbledon semi-finalist Ostapenko, who will next face Tatjana Maria after she won 6-3, 7-5 against Tereza Valentova, was just pleased to be back in action on Thursday.
"I didn't think I was going to play today," she said. "After the match yesterday at 10:00 pm, I had heat stroke, so I felt really bad."
Follow the women's side at Eastbourne here.
Draper marches on
In the parallel ATP event, former world number four Jack Draper continued his promising return from injury under the guidance of Andy Murray with a 6-1, 6-4 victory over Gabriel Diallo that saw him into his first semi-final since Queen's last year.
Draper will now play France's Ugo Humbert after he defeated compatriot Quentin Halys 6-3, 6-4.
And Draper won't be the only British man in the semi-finals after lucky loser Toby Samuel enjoyed the biggest win of his professional career with a 6-3, 6-4 success against eighth seed Juan Manuel Cerundolo.
Samuel will now face Belgium's Zizou Bergs after he defeated British qualifier Jan Choinski 6-3, 6-3.
