By Simon Cambers
ROME (Reuters) - Serena Williams enjoyed an easy passage into the semi-finals of the Italian Open on Friday when her opponent Flavia Pennetta retired with a right wrist injury.
The Italian, ranked 21, had treatment at 3-0 down and decided to quit when Williams was leading 4-0 and 40-0.
"It's never nice to go through like that but it is nice to have kind of a semi-day off," Williams told reporters. "I've been playing every day for I don't know how many days so it definitely helps."
Williams, who won last week's title in Madrid, will play China's Li Na in the last four after she beat Dominika Cibulkova of Slovakia 6-1 7-6.
"Li's the defending French Open champion and a great all-court player so it's going to be a really good test for me," Williams said.
Having also won in Charleston, Williams feels confident heading into the French Open, which begins on May 27.
"I feel like I can play on any surface," she said. "I've played on ice before and on water. I feel like I can play on anything."
After being sidelined from July 2010 to June 2011 because of a blood clot in one of her lungs, the world number six said she was now feeling more confident than last summer, when she won back-to-back tournaments before losing in the final at the U.S. Open.
"I definitely feel better because I've played more tournaments," she said. "Last summer my lungs had not recovered completely."
Eighth seed Li let slip a 5-2 lead in the second set before holding on to take the tiebreak 7-4.
Holder Maria Sharapova was due to play Venus Williams later on Friday while Wimbledon champion Petra Kvitova was scheduled to face Angelique Kerber of Germany.
(Editing by Pritha Sarkar)