"Great to hear that USADA is investigating some of SI's claims. I look forward to being vindicated," the seven-time Tour de France winner said.
Armstrong is currently competing in the Tour Down Under in South Australia and has refused to comment on the Sports Illustrated report, other than to say he has nothing to worry about "on any level" from its claims. He would not speak to reporters after the fourth stage on Friday and could not be contacted later in the evening.
"While we can't comment on the specifics of the investigation that UCI and USA Cycling asked us to open last year, we value the confidence all athletes have in the process to ultimately reveal the truth, preserve the integrity of sport, and protect the rights of clean athletes," USADA CEO Travis Tygart wrote in an e-mail to The Associated Press.
Sports Illustrated reported that when Italian authorities raided the home of Armstrong teammate Yaroslav Popovych last November in Italy, they found texts and e-mails linking their Radio Shack team with Dr. Michele Ferrari as recently as 2009.
Armstrong publicly severed his connection with Ferrari in 2004, amid accusations the doctor aided another rider in using performance-enhancing drugs. Ferrari was convicted and then later cleared of criminal charges on appeal.
Armstrong spokesman Mark Fabiani called the Sports Illustrated report "old news from the same old, discredited sources."
A federal grand jury in Los Angeles has been hearing evidence for months on cheating in professional cycling.
The investigation turned toward Armstrong -- and several of his associates have testified -- since former teammate Floyd Landis accused him of systematic doping.
Armstrong won the Tour de France every year from 1999 to 2005.
Armstrong came in 85th of 131 riders in the Tour Down Under on Friday, and is 81st overall after four stages. He is 3 minutes, 56 seconds down on the race leader Cameron Meyer in his last professional race outside the U.S.