By Sonia Oxley

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 Center Court against France's Aravane Rezai 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."

Venus is indeed back after both sisters returned to the circuit last week at Eastbourne, where Serena was closely watched by her peers, fans and -- above all -- her mother.

"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."

Serena was knocked out in the second round of Eastbourne by Vera Zvonareva, the Russian whom she beat in last year's Wimbledon final, but was happy with where her game was at after such a long absence even if there was room for improvement.

"I didn't travel with my serve to Eastbourne, so I hope it came with me," she said.

"I hope he was in my luggage coming to Wimbledon, because I've missed him."

"Hopefully I'll have a good serve this week," added Serena, before amending that to "fortnight" as if anyone had any doubt of the champion's intentions.

(Reporting by Sonia Oxley; Editing by Alan Baldwin)