BUENOS AIRES, Nov. 15 (Xinhua) -- Former world No. 3 Juan Martin del Potro of Argentine has withdrawn from an exhibition match with 20-time Grand Slam winner Roger Federer because of ongoing knee problems.

The 31-year-old will be replaced by German Alexander Zverev for Wednesday's clash at Parque Roca in Buenos Aires, organizers said on Friday.

Del Potro has not played since June, when he fractured the patella in his right knee for the second time in nine months during the Queen's ATP 500 tournament in London.

"I'm still not able to move freely, it's hard for me to run," the Argentine said in a video posted on social media. "It has been a frustrating year ... [but] I hope to return as soon as possible."

The 2009 US Open champion has been beset by injuries in recent years and long periods on the sidelines saw his career ranking plummet to 1,045th place in 2016.

Last year, he won two ATP titles and reached the final of the US Open, where he was beaten by Novak Djokovic.