World No. 21 Alexandr Dolgopolov announced that he will miss the entire US Open Series and Davis Cup due to a knee injury that may require surgery.

The Ukrainian, who lost to Tobias Kamke in the third round of the ATP 500 event in Hamburg this week, made the announcement on his official Instagram account Saturday.

"I am really unhappy to announce that I injured my knee in Hamburg and I will probably need a surgery and will be out for 4-6 weeks," he wrote on on the photo-sharing site. "Disappointing,specially not playing the great event in Umag, the Us series and Davis cup play offs:((( hopefully il be back ASAP."



The 25-year-old fell out of the top 50 in 2013, but has been inching his way back towards the top 20 this season. He reached the semifinals of the BNP Paribas Open in Indian Wells this year, upsetting Rafael Nadal along the way, and the quarterfinals of both the Sony Open in Miami and at the Queen's Club in London.

Dolgopolov also reached the fourth round of the US Open in 2011.