Defending champion Simona Halep was knocked out of the French Open 6-2 6-4 by 17-year-old American Amanda Anisimova in a quarter-final shock on Thursday.

The third seed from Romania had been looking to become the first woman to retain her title at Roland Garros since Belgian Justine Henin won three times in a row from 2005-07.

Anisimova, the first player born in the 2000s to reach the last eight at a Grand Slam, can become the first teenager to lift the trophy here since Iva Majoli of Croatia prevailed in 1997.

"This is honestly more than I could ever have asked for," the teenager said on court at the finish. "That was one of the best matches I have ever played." 

In the last four, she will take on another first-time slam semi-finalist in Australia's Ashleigh Barty.

The eighth seed is thought of as more of a threat on faster surfaces, but she has been in superb form this season and has carried that onto the clay.

Barty was a 6-3 7-5 winner over 14th seed Madison Keys, taking her second opportunity to serve out the victory.