The Williams sisters had to utilise all their resolve and battling prowess to progress into the US Open fourth round.Defending champion Serena snatched the first set against Maria Jose Martinez Sanchez and then survived a second-set fightback before prevailing 6-3 7-5.Third seed Venus produced an erratic display against Magdalena Rybarikova and, after struggling on serve throughout, eventually won 6-2 7-5.Serena now faces Daniela Hantuchova, with Venus up against Kim Clijsters.Number two seed Serena, who accused Martinez-Sanchez of cheating when they met at the French Open this year, was relieved after a slight wobble in the second set."You can never underestimate anyone," said the second seed, who attended her news conference wearing a T-shirt emblazoned with the message, "Can't spell dynasty without nasty"."Some people, some days they have great days. I was just happy to keep getting through my match."I got nasty, but to myself. I was just screaming to myself because I wasn't very happy with my performance."Slovakian Hantuchova saw off American Vania King 6-2 6-3 earlier on Friday with an impressive performance. The number 22 seed was never in trouble as she struck 25 winners on her way to four breaks of serve, with King unable to live with Hantuchova's guile and speed across the court.Clijsters, competing at Flushing Meadows for the first time since she won the title in 2005, continued her fine comeback to win the battle of the Belgians and reach round four.The 26-year-old, who recently returned after leaving tennis to get married and have her first child in 2007, took just 62 minutes in beating compatriot Kirsten Flipkens 6-0 6-2.Equally comfortable in victory was seventh seed Vera Zvonareva, who dispatched fellow Russian Elena Vesnina 6-2 6-4 in 85 minutes to set up a fourth-round meeting with Flavia Pennetta.The 27-year-old Italian underlined her US Open credentials with another rapid-fire win.Pennetta, seeded number 10, demolished Canada's Aleksandra Wozniak 6-1 6-1, meaning she has now lost only four games in three rounds. Appearing at the US Open for the sixth time, Pennetta has made significant strides forward this year, building on an excellent 2008 when she reached the last eight at Flushing Meadows and the fourth round at the French Open."Tennis, it's a very strange sport, because for one point everything can change," she said. "You have to be very mentally focused at all moments."Pennetta's compatriot Francesca Schiavone, seeded 26th, also progressed, securing a shock victory over Belarussian eighth seed Victoria Azarenka.Azarenka, who some had been tipping as an outsider for the title, looked on course for an expected win when she took the first set, but was undone by a total of 30 winners from Schiavone as the Italian won through 4-6 6-2 6-2.Schiavone will now play China's Na Li after the 18th seed beat Maria Kirilenko 6-4 6-2.Source: BBC