NEW YORK (AFP) - Rafael Nadal and Ana Ivanovic, who are playing their first US Opens as the top seeds, take to the court in day four against a pair of qualifiers.

Both have had limited success in the past in the final Grand Slam of the season at the Billie Jean King National Tennis Centre. In five tries Spain's Nadal has been unable to get past the quarter-finals and Ivanovic, of Serbia, has never been beyond the fourth round.

Nadal faces American qualifier Ryler DeHeart in a second round match on the main Arthur Ashe Stadium. The reigning French Open and Wimbledon champ could be on a collision course for a semi-final showdown with Scot Andy Murray who is also in action on Thursday.

Nadal recently took over the number one world ranking, surpassing Roger Federer who had held the position for a record 237 weeks.

Ivanovic squares off against France's Julie Coin in a second round match on the Louis Armstrong court.

Number six seed Murray is looking to improve on his fourth round finish at Flushing Meadows in 2006. He is facing France's Michael Llodra who won titles earlier this year on hardcourt in Rotterdam and Adelaide.

The Williams sisters are also in action Thursday with Serena getting the night session nod over seventh-ranked Venus.

Their Davis Cup teammate Andy Roddick showed off his devastating serve and precision forehand in Wednesday's night match, firing 15 aces en route to a 6-2, 6-2, 6-2 demolition of Fabrice Santoro.

While serving for the match, Roddick put an exclamation point on his victory by smacking a 220 kilometres (140 miles per hour) serve that whizzed past the Frenchman's head, sending him ducking for cover.

Santoro glared at Roddick then lowered his racket and put his hands on his hips and motioned for Roddick to make his final serve. An obvious upset Santoro made no attempt to return Roddick's game-ending serve.

Santoro said afterwards it was unnecesary while Roddick said the bad feelings took the lustre off his win.

"I was really excited about the way I was playing," Roddick said. "To end it like that, it was a little disappointing."

Roddick won 89 percent of his first serve points against Santoro who didn't play for over a month heading into the US Open.

"I wanted to try to bully the ball and try to get on top of him early," Roddick said.

Santoro showed the rust of not playing regularly and appeared a shadow of the player who beat 14-time Grand Slam winner Pete Sampras in three of their seven career meetings.