Emma Raducanu's "challenging year" suffered another hiccup after the former US Open champion pulled out of her scheduled comeback at an exhibition tournament next month in Macau.

The Briton, who turned 21 on Monday, has not played since April after undergoing operations on both wrists and also an ankle.

She had targeted the Macau Tennis Masters on December 2-3 for her return but her name was missing when the final field was announced on Monday, with organisers saying "injuries (led) to a revised line-up".

Raducanu stunned the tennis world two years ago when she won the US Open as a qualifier aged 18.

But she has struggled for form and fitness since and is now ranked 289 in the world.

She is however back in training following her long injury layoff and is reportedly targeting the beginning of next year for her comeback.

The Australian Open begins on January 14.

"Regardless of a challenging year there's a lot to be grateful for," she wrote on Instagram as she celebrated her birthday.

"Looking forward to the next chapter."