Serena relying on lung-power

The 13-times grand slam winner suffered potentially fatal blood clots in a lung earlier this year, only returning to action last week after nearly a year on the sidelines due to a combination of health issues and injuries.

"I have to do things differently because I had to expand my lung capacity, because I lost a little bit of my lung," the 29-year-old American told a news conference on the eve of the start of the grasscourt grand slam at the All England Club.

"I just had to get my lungs into better shape. I'm probably actually in better shape running-wise than I was before."

Williams opens her bid on Centre Court against France's Aravane Rezai on Tuesday with a possible fifth Wimbledon crown far from her thoughts after a year where the lung problems followed two operations on a sliced tendon in her foot.

"My thought process is just to play the best I can and to be positive," she said. "I'm happy to have been here ... the fact that I can even compete and be in a position I wasn't sure I'd have a chance to be at again is more than enough."  During the bad times over the past year, Williams took some comfort from the fact her sister and five-times Wimbledon champion Venus was also enduring a prolonged injury layoff.

"It's been good because - sometimes, I hate to say it - but when you're down, it's always good to see someone down with you," said Serena with a laugh. "And she was down with me. I was like, 'Ha-ha'. Maybe we'll come back together."

"My mom was so worried about me," she said. "I keep telling her I'm OK. She's like, If you feel anything, just stop. Come off the court. I'm like, Mom, the doctor said I would be OK."