Martina Trevisan sprung the latest shock of an eventful Roland Garros which has seen a succession of seeds dumped out.
"I came here two weeks ago to play the qualifiers and today I'm here in the quarter finals. I can't believe it," said Trevisan, who will break into the top 100 of the WTA rankings on Monday.
"I'm really honoured to play on this court with Bertens; she's an incredible player."
After a mastering the opening set, Trevisan went 3-0 up in the second before Bertens stepped up a gear to break back.
Trevisan, however, held her nerve and wrapped up victory with an exquisite lob on her third match point on Bertens's serve.