Serena Williams into Rome Open semi-finals after match retired

Serena Williams has moved swiftly into the Rome Open semi-finals after the match between the American and Flavia Pennetta was forced to retire.

The former world number one was the dominant competitor in the early stages as her Italian opponent continued to struggle.

The contest was temporarily stopped in the third game of the opening set but was officially called off at 4-0 to Williams with Pennetta unable to continue.

Williams will face either Li Na or Dominika Cibulkova in the semi-final stage of the tournament.