Defending champion Maria Sharapova and third-seeded Victoria Azarenka endured rocky rides into the French Open last-16 on Saturday while the United States saw four women make the second week. World number two Sharapova clinched a 6-1, 7-5 win over China's Zheng Jie, the world number 43, in a third round clash which witnessed 11 breaks of serve.

The second seeded Russian will take on America's Sloane Stephens, who she beat comfortably on clay in Rome two weeks ago, for a place in the quarter-finals. But the 26-year-old has work to do after firing down 34 unforced errors, an ugly tally which was at least partially soothed by her 25 winners.

She was also furious over a bad line call which put her 1-4 down in the second set even though TV replays showed her ball to have landed on the line. "All the other Grand Slams have Hawk-Eye," said Sharapova, in reference to the instant replay system. "Why don't we have a system like this? I mean, is it a money concern? I don't think so." Sharapova, playing in her 11th French Open, had reached the third round for the loss of just nine games and seemed on course for another routine win on Saturday.

Four breaks of serve helped her to the first set. However, two breaks took the 29-year-old Zheng out to a 4-1 lead in the second before the champion pulled level at 4-4, pumping up the volume at the same time. Zheng broke back for 5-4 but she was unable to serve out the set as Sharapova levelled again at 5-5. The world number two held for 6-5 and took the tie when Zheng netted a return.

Azarenka overcame 10 double faults and 31 errors on her way to defeating France's Alize Cornet, 4-6, 6-3, 6-1. The 23-year-old Australian Open champion reeled off the last six games of the match and goes on to face Italy's 2010 champion Francesca Schiavone. "I really felt that I left my serve at home," said Azarenka, who has never got beyond the quarter-finals in Paris. "But if I can win serving like this, that's pretty remarkable."

The 23-year-old Cornet, fresh from a third career title in Strasbourg on the eve of Roland Garros, had the initiative in the deciding set when she broke to lead 1-0. But Azarenka stepped on the gas, winning six games on the bounce to take victory. "She a machine, a juggernaut," said Cornet. "She plays the same from the first to the last point." The United States will have four women in the fourth round.

Top seed Serena Williams made sure of her place on Friday and on Saturday she was joined by Sloane Stephens, Bethanie Mattek-Sands and Jamie Hampton. Stephens, the 17th seed, reached the last 16 for the second successive year with a 6-4, 6-7 (5/7), 6-3 win over New Zealand's Marina Erakovic. "I felt like I played well the whole time. Like my coach always says, you have to respect the other player and what they're doing," said Stephens, who has won three matches in succession for the first time since reaching the Australian Open semi-finals in January.

Mattek-Sands, the conqueror of 2011 champion Li Na in the second round, made the last 16 for the first time with a 4-6, 6-1, 6-3 win over Argentine qualifier Paula Ormaechea. Mattek-Sands will face next Russian 12th seed Maria Kirilenko. Hampton, the world number 54, achieved her first win over a top 10 player when she held her nerve after wasting two match points to beat Czech seventh seed Petra Kvitova, 6-1, 7-6 (9/7).

The 23-year-old from Alabama will face either Australian ninth seed Samantha Stosur, the 2010 runner-up, or Serb 18th seed Jelena Jankovic, a three-time semi-finalist in 2007, 2008 and 2010. Schiavone, now at 50 in the world, ended French hopes in the women's singles by putting out 13th seed Marion Bartoli, 6-2, 6-1.