The Wimbledon and French Open champion raised his game when challenged to defeat Serbia's 71st-ranked Viktor Troicki 6-4, 6-3, 6-0, giving the Spaniard his 41st triumph in his past 42 starts.

'I have started playing better,' Nadal said. 'Today I served very well. I have more confidence and I hope to continue to improve my tennis. I know I have to keep improving my tennis if I want to win.'

Nadal can match his best US Open showing to date by winning his next match over Sam Querrey, the 55th-ranked American who ousted Croatian 14th seed Ivo Karlovic 7-6 (7/5), 7-6 (7/5), 6-2.

'I have to play very well if I want to win,' Nadal said. 'I will just focus on my next match because every match is difficult. He's playing very well.'

Seventh-seeded Venus and fourth-seeded Serena Williams also advanced to the fourth round, the US sisters who met in the Wimbledon final moving one step closer to a quarter-final showdown at the year's last Grand Slam championship.

British sixth seed Andy Murray rallied past Austrian Jurgen Melzer 6-7 (5/7), 4-6, 7-6 (7/5), 6-1, 6-3, booking a date with Swiss 10th seed Stanislas Wawrinka in the round of 16 after battling for three hours and 52 minutes.

'He was playing unbelievable,' Murray said. 'He was putting serves bang on the line. I really didn't get any rhythm. I just kept fighting. Once I broke him in the third set that's when I started to get my rhythm.

'This is why you put in all the work. Hopefully I can keep it going.'

France's Gael Monfils pulled off the biggest surprise so far on the men's side by ousting Argentina's seventh-seeded David Nalbandian 6-3, 6-4, 6-2.

Monfils will next face either US ninth seed James Blake or American Mardy Fish.

Nadal, whose only loss in his current run came to Serbia's third-ranked Novak Djokovic in the Cincinnati semi-finals, thrilled a record crowd of 37,380 at Arthur Ashe Stadium.

After breaking Troicki in the last game of the first set, Nadal surrendered a break by netting a forehand to give his rival a 2-1 lead.

But Nadal responded in the sixth game by swatting a running forehand winner past Troicki to break back level at 3-3 and never lost another game.

'That gave me a lot of confidence that I could win,' Nadal said. 'He was playing well in that moment but after that I started playing better and he made some mistakes.'

After capturing Olympic gold at Beijing, Nadal is trying to become only the fourth man to win three Slam titles in a row in the 40 years of the Open era, joining Roger Federer, Pete Sampras and Rod Laver.

Nadal, seeking his ninth title of the year, defeated Federer in the French Open and Wimbledon finals and moved past him atop the rankings on August 18 after being second for a record 160 weeks.

Querrey, who won his first career title this year at Las Vegas, fired 20 aces and made only 13 unforced errors without surrendering a service break to dispatch Karlovic and earn his first trip to the round of 16 at a Slam.

Nadal beat Querrey 6-7 (5/7), 6-2, 6-3 in 2006 at Cincinnati in their only prior meeting.

'I've gotten a lot better since then. He has too,' Querrey said. 'I can go out there and rip away. I have nothing to lose. Pretty much it's just step up and rip it.'

Venus Williams advanced by beating Ukraine's Alona Bondarenko 6-2, 6-1 on Saturday, the seventh seed needing only 56 minutes to bounce Bondarenko for the second year in a row. Williams fired nine aces and 32 winners.

'I felt like I had to play my game, serve huge, and that served me well,' Williams said. 'She is very consistent.'

Poland's ninth-seeded Agnieszka Radwanska is up next for Venus. Radwanska, who eliminated Slovakia's Dominika Cibulkova 6-0, 6-3 in 76 minutes, beat Williams in 2006 at Luxembourg in their only prior meeting.

'I'm looking forward to evening the score,' Williams said.

Eight-time Slam champion Serena Williams defeated Japan's Ai Sugiyama 6-2, 6-1. Serena will have to defeat either Italy's Tathiana Garbin or France's Severine Bremond to reach the final eight.

'I'm definitelty not at my best but I hope to get there,' Serena said. 'I think she's playing really well. I'm glad she feels she's playing her best.'

Sunday's top matches find defending champion Roger Federer playing Czech Radek Stepanek, fellow second seed Jelena Jankovic facing Denmark's Caroline Wozniacki and Serbian third seed Novak Djokovic meeting Croatia's Marin Cilic. -- AFP