Flavia Pennetta stuns Maria Sharapova at Indian Wells

Italy's Pennetta, the defending champion at Indian Wells, left the court momentarily after breaking down in tears following her first set loss.

When the 15th seed returned, she seized the momentum, going on to win 10 of the last 13 games to blaze past the two-time champion Sharapova and into the last eight.

Sharapova said she was unaware of Pennetta's emotional outburst and had struggles of her own, the Russian finishing with 42 unforced errors and 11 double faults.

"Maybe I wasn't playing my best tennis but I was competitive enough," Sharapova said. "I just wasn't able to step up in the key moments."

Earlier, top seed Serena Williams overcame a wobbly start to see off a determined Sloane Stephens 6-7(3) 6-2 6-2 and move into the last eight.

Williams, playing her third match at Indian Wells after deciding to end a 14-year boycott of the event where she had suffered alleged racist abuse, enjoyed plenty of support but struggled early to find her form.

Stephens, who reached the quarter-finals a year ago, looked ready to return to the last eight after breaking Williams twice to open the match but in the end there was no denying the world number one as she extended her winning run to 14 matches.

"I don't feel like I need to come here and win the trophy," Williams told reporters. "I feel like this is more than I have ever imagined doing. I never imagined even having an opportunity to come back.

"I feel like I have been a champ here, and I'm really happy with every match that I win and every result. For me it's all icing on the cake."

On another sun-bathed afternoon, Stephens got the contest off to a bright start by charging 3-0 ahead.

But Williams, who has not lost since last year's WTA Finals in Singapore, quickly got her game in gear, sweeping through the next four games before her opponent could again hold serve.

Displaying plenty of grit of her own, Stephens dug in to force an opening set tiebreak that she dominated.

Playing with a greater urgency, Williams broke Stephens to open the second and would break her compatriot again to go up 5-2 before serving out the set to level the match.

The third set followed a similar pattern as Williams recorded the early break to secure a 2-0 advantage. She broke again in the seventh game and held serve to close out a challenging test in two hours, six minutes.

In other matches, third seeded Romanian Simona Halep rolled past 14th seeded Czech Karolina Pliskova, while sixth seed Eugenie Bouchard blew a late lead and was upset by Lesya Tsurenko.

There were also victories for 12th seeded Spaniard Carla Suarez Navarro, who beat Briton Heather Watson, and 18th seed Jelena Jankovic, who prevailed in three sets over Belinda Bencic.

Switzerland's Timea Bacsinzky and Germany's Sabine Lisicki both won to complete the quarter-final line-up.