RIO DE JANEIRO (Reuters) - Alexander Zverev, the rising star of German tennis who beat Roger Federer in the Halle Open semi-finals in June, pulled out of the Rio Olympics on Sunday due to fitness problems.

"During my last two ATP matches in Washington and Toronto, I did not feel 100 per cent and after several consultations with doctors and my team, we have decided that I must withdraw from the Olympics," the 19-year-old said on Instagram.

"I hope to be back in full form for the ATP Masters 1000 in Cincinnati and wish my German team-mates all the best in Rio."

Zverev, who lost the Halle final to compatriot Florian Mayer after beating 17-time Grand Slam champion Federer, is ranked 25th in the world and would have been seeded at the Olympics.

The big-serving teenager lost in the semi-finals in Washington to Gael Monfils of France and the first round in Toronto last week to Taiwan's Lu Yen-Hsun.

Germany have Philipp Kohlschreiber, Dustin Brown and Jan-Lennard Struff entered in the Rio singles event.