MASON, Ohio, (Reuters) – Former world number one Andy  Roddick said a virus was to blame for his recent fitness  problems but expects to be back in good shape for the U.S. Open.

Roddick pulled out of this week’s Toronto Masters citing an  illness and after a lop-sided loss to Frenchman Gilles Simon in  the recent Washington Classic third round he complained of  feeling lethargic.

The 27-year-old American will compete in next week’s  Cincinnati Masters after tests showed he had been suffering from  mononucleosis, a viral illness.

“I pulled out last week on the advice of doctors. I got some  blood work done in Washington, and then again in Texas, and one  of them came back positive. I’ve been dealing with a mild, mild  case of mono for the last couple of months that I wasn’t really  sure about,” he told reporters.

“They think I’m fine now. They think I’m most of the way  through it, if not all the way through it. It’s kind of a mental  relief opposed to wondering why and if I was out of shape,” he  said.

“It’s nice to just have a little bit of clarity moving  forward. It’s not something that’s going to affect me, anything  super serious.

Roddick, the 2003 U.S Open champion, won in Brisbane this  year and he was playing well until March but he has since  slipped outside the top 10 in the world rankings.

The American said he had been feeling out of sorts for the  past two months and had been skipping some physical training.

“Normally I enjoy training all day and running all day and  doing all that stuff. I opted out of probably four or five  workouts this summer that I’ve never done that before,” he said.