Reigning US Open champion Novak Djokovic was disappointed in the doping downfall of American cyclist Lance Armstrong, whose seven Tour de France titles have been stripped by the US Anti-Doping Agency.

Armstrong, who won the world’s most famous cycling race every year from 1999 through 2005, said on Thursday he would not fight charges by US anti-doping officials that his success came as part of a doping conspiracy.

“When I heard that story, and many others, I’m disappointed as an athlete because I know how much it takes to get to where we are and on the top of our own sport, how much sacrifice, commitment, hard work,” Djokovic said.

“In the end, we are all seeking to have pure sport. I’m happy that in tennis we do not have that many cases and we are trying to keep that going to keep tradition and to protect the integrity of the sport.

“That’s something that sends a strong message about our sport also to young kids because they look up for heroes and they look for role models.”

Djokovic, 25, has been that to the youth in his native Serbia and beyond for years and his fame could rise even higher with a repeat as US Open champion, which would bring him $US2.9 million ($A2.8 million) in prize money.

“I do feel physically stronger and better prepared than I did last year,” Djokovic said.

“Mentally, I had some ups and downs throughout the season, but I think that was maybe expected in a way. It’s normal to have ups and downs.”

AAP djw/