The diminutive Italian twice broke her Dutch opponent, who has been playing with a back injury all week.
"I had a lot of pain. I wake up this morning I it was worse than yesterday. I feel an unlucky lucky loser," Bertens, whose first serve was down at 44 per cent, said.
The 21-year-old six-footer turned professional in 2009 but her breakthrough year was 2012 - she won her one WTA title in Fes title in April - and she will climb towards the top 50 from her current position of 65 after a good week prior to this point.
Errani, who won their one previous meeting on the grass of Den Bosch in 2011 in straight sets, has only had to play one complete match on her way to the final.
The 25-year-old, ranked seventh in the world, is looking for a seventh career title.
1-Sara Errani (Italy) beat Kiki Bertens (Netherlands) 5-0 (Bertens retired)
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