(CNN) -- Serena Williams moved a step closer to defending her U.S. Open title, although she had to produce her best tennis to see off Spain's Maria Jose Martinez Sanchez.
Serena Williams shows her relief after clinching victory over Maria Jose Martinez Sanchez.
The 27-year-old American won 6-3 7-5 to secure a fourth-round showdown with Slovakian Daniela Hantuchova, who eased past another American, Vania King, 6-2 6-2.
Second seed Williams had to fight back from 1-3 down in the final set to claim victory and praised her opponent afterwards. "She was serving really well especially in the second set, but I just kept on fighting," the American told reporters.
"It's not often that I play someone that serves and volleys like that.
"Playing doubles here helped with that. I got super-pumped when 1-3 down in the second set as I knew I had to recharge my batteries."
Serena will be joined by sister Venus, who survived a tough challenge from Magdalena Rybarikova of Slovakia, 6-2 7-5.
Venus Williams next faces former world number one, Kim Clijsters. The Belgian ace continued her fine run on her grand slam comeback on Friday, brushing aside compatriot Kirsten Flipkens, 6-0 6-2.
In-form Italian Flavia Pennetta is also through to the last 16 after crushing Canada's Aleksandra Wozniak 6-1 6-1.
The 27-year-old 10th seed, who has now lost just six games in three rounds, was joined by compatriot Francesca Schiavone, who upset eighth seed Victoria Azarenka of Belarus 4-6 6-2 6-2.
Pennetta won titles at Palermo in July and Los Angeles last month and comes into New York in the best form of her career.
She defeated Romania's Edina Gallovits 6-0 6-4 in the first round and then whitewashed India's Sania Mirza 6-0 6-0 in the second round.
Pennetta will next play seventh seed Vera Zvonereva for a place in the quarterfinals, where she could run into favorite Williams.