Serena Williams has revealed that her first-round victory over Mandy Minella at Wimbledon was tougher than she had expected.

Last year's winner enjoyed a successful start to her 2013 campaign as she came through 6-1 6-3 on Tuesday.

"It was really special coming out as defending champ," the American told BBC Sport. "Winning here last year was a great moment for me, but also winning the Olympics here means it's really special.

"I played so many matches on clay this year and that meant the transition to grass was a little harder than I expected."

Williams will take on French world number 100 Caroline Garcia in the next round.