Duncan has picked up two fourth placings at the latest round at the Assen track in the Netherlands.
With the result she slipped off the top ranking and is lying second with one round left, next weekend in France.
Duncan is now two points behind the new leader in the women's world championships.
The new leader is Dutch rider Nancy van de Ven, who made the most of racing in front of her home crowd, winning both races at Assen.
Duncan's team boss Josh Coppins said it had been a tough weekend for Duncan.
``Obviously it was a bad weekend and, to be honest, she didn't ride well and made too many mistakes. This is making the championship tight and putting her under a lot of pressure,'' Coppins said.
``The thing now [is] pressure goes on the other three top riders too because whomever wins in France can win the title.
``It has been a little bit tricky because she tore the meniscus (cartilage) in her knee in the GP in the Czech Republic and there was only limited training that we could do after that.
``But that's no excuse because she still did enough training. She was strong enough to win and her knee didn't bother her on race day. She will still require surgery after the French GP.
``Courtney is obviously pretty disappointed because she's made it hard for herself.''
Duncan was philosophical about her latest set-back.
``This weekend was a difficult one for me. It was just one of `those' weekends where I felt a little bit off,'' she said.
``I struggled to ride like I can. I am looking forward to the final round. The speed is there, my starts are there, we've just got to sort a few things out and I am sure that everything will be fine. Not every weekend is good, and this was just one of `those' weekends.
``It was by far the worst race of the season but it is what it is. We've already turned the page. I'm taking this as motivation and moving forward.''
Duncan will spend the week training and getting treatment for her injury before the final round at the French GP at Villars-sous-Ecot this coming weekend.
