Zhang dropped the opener but recovered to clinch a hard-fought 18-21, 21-13, 21-13 success over Kjærsfeldt at the Saarlandhalle in the German city of Saarbrücken.

The American player will take on Lee Chia Hsin of Chinese Taipei in the last eight.

Lee set up a meeting with the top seed after she beat England's Chloe Birch 21-8, 21-19.

Zhang was joined in the quarter-finals by second seed Nichaon Jindapol of Thailand, who remains on course to face the American in the final after she came through her round of 16 clash with Canada's Michelle Li.

Jindapol, a three-time Southeast Asian Games gold medallist, was also forced to do it the hard way as she came back from a game down to win 18-21, 23-21, 21-7 in a match which lasted seven minutes shy of an hour.

Her opponent in the last eight will be Chiang Mei Hui of Chinese Taipei, who progressed by virtue of a 21-17, 21-19 victory against Germany's Yvonne Li.

Jindapol's compatriot Pornpawee Chochuwong was not able to follow her team-mate into the next round as the 19-year-old seventh seed was beaten 21-19, 13-21, 22-20 by Indonesia's Dinar Dyah Ayustine.

In the men's event, European champion Rajiv Ouseph of England crashed out as he was beaten by Dutchman Mark Caljouw.

Ouseph, seeded second but who was the favourite to take the title at the BWF Grand Prix Gold level event after the withdrawal of Denmark's Anders Antonsen, lost 21-16, 11-21, 21-14.

The tournament continues tomorrow.