Belgium's former world number one Clijsters withdrew from the clash with Poland's Radwanska after feeling a stomach muscle strain before the clash in the Netherlands.
She will face eighth seed Nadia Petrova in the final after the Russian ended Kirsten Flipkens's run with a 6-4 6-2 semi-final win.
In the men's draw, German Philipp Petzschner beat Belgium's Xavier Malisse 7-5 7-5 to reach the final, where he will play top seed Ferrer after he overcame a second-set wobble to eke out a 7-6 3-6 6-1 win over France's Benoit Paire.
The 29-year-old Clijsters, now ranked 53rd in the world, said she did not want to jeopardise playing at Wimbledon next week, which is set to be her last appearance at The Championships in SW19 before retirement.
"This is a decision taken with my head, not my heart," Clijsters said. "But preventative withdrawing from this tournament is the right decision. I am definitely going to play at Wimbledon."
Her physiotherapist added that she had felt the tweak before her quarter-final win over Francesca Schiavone on Thursday.
Later on Friday, Flipkens lost out to Petrova for a place in the final. The Belgian had been forced to complete her rain-delayed straight-sets win over Italy's Roberta Vinci in the quarter-finals earlier that day.
That match had been halted at 6-4 6-5 due to rain on Thursday, with Flipkens winning the tie-break 7-5.