Rome, Italy (SportsNetwork.com) - Serena Williams pulled out of her scheduled third-round match, while French Open champion Maria Sharapova and Roland Garros runner-up Simona Halep reached the quarterfinals at the Italian Open, a clay-court French Open tune-up.

The world No. 1 superstar Williams cited an elbow injury that began during last week's tournament in Madrid as the reason for her withdrawal.

She was scheduled to face qualifier fellow American Christina McHale on Thursday.

"It was not an easy decision, I talked to my coach who said it would not be a smart thing to play," said Williams. "I hate quitting. But this is not quitting, it's just making a better decision."

The 33-year-old Williams was the two-time defending Rome champion and also won the title here in 2002.

"I love playing in Rome and have a lot of friends here," Williams added. "Pulling out is not fun but everyone is adult and understands the importance of someone doing the best they can at this stage of their career. It is nothing personal."

Williams will be the top seed in Paris just 10 days from Thursday. She lost in the second round last year at Roland Garros after capturing her second career French Open title in 2013.

Meanwhile, the second-seeded Halep whipped Williams' 14th-seeded older sister, Venus, 6-2, 6-1, while the third-seeded former No. 1 Sharapova subdued Serb Bojana Jovanovski 6-3, 6-3 on Day 5 at Foro Italico. Venus was the Rome titlist in 1999 and runner-up in 1998.

Sharapova titled here back-to-back in 2011 and 2012.

Hot fourth-seeded Czech Petra Kvitova charged on with 6-3, 6-2 spanking of 16th-seeded former No. 1 Jelena Jankovic, a two-time Italian Open champion and former runner-up.

Kvitova was last week's clay-court champ in Madrid.

Unseeded former world No. 1 Victoria Azarenka moved on with a hard-fought 6-4, 5-7, 6-1 victory over Romanian Irina-Camelia Begu in Thursday's nightcap. Azarenka was the 2013 Rome runner-up to Serena.

In other action involving seeds, No. 10 Carla Suarez Navarro upset No. 6 Wimbledon runner-up Genie Bouchard 6-7 (2-7), 7-5, 7-6 (9-7) and Romanian qualifier Alexandra Dulgheru upended No. 8 Russian Ekaterina Makarova 6-4, 6-3.

Another third-round result saw Russian qualifier Daria Gavrilova take out rising Swiss Timea Bacsinszky 6-4, 7-6 (7-0).

Friday's quarterfinals will pit Halep against Dulgheru; Sharapova versus Azarenka; Kvitova against Suarez Navarro; and McHale versus Gavrilova.

Sharapova and Azarenka have split 14 career matchups, but the Russian has won the last three, including a tussle at Indian Wells earlier this season. Sharapova also topped Azarenka in a quarterfinal here in Rome in 2011 and has captured all three of their clay-court WTA encounters.