NEW YORK (Reuters) - Top-seeded Spaniard Rafael Nadal plans to step up his intensity when he bids for a place in the third round of the U.S. Open on Thursday.

The newly crowned world number one, looking to win his third grand slam title of the year, was below his best in the opening round but promised to up his game when he faces American qualifier Ryler De Heart, ranked 261.

"Probably I'm a little bit more tired than usual," Nadal said, having arrived in New York later than normal for the year's final grand slam tournament after winning the gold medal at the Olympic Games in Beijing.

"But the thing is to try to be good, mentally and physically, because I think I am playing good tennis."

"The first round always is difficult, and hopefully next round I play much better."

Women's top seed Ana Ivanovic of Serbia, who survived a tough battle in round one, faces French qualifier Julie Coin, while both Williams sisters are in second-round action.

Fourth seed Serena takes on Russian Elena Vesnina while Wimbledon champion Venus meets Paraguayan Rossana De Los Rios.

(Editing by Mark Lamport-Stokes)