Armstrong confessed in a televised interview with chat show host Oprah Winfrey this week to using banned performance-enhancing substances to establish himself as one of the biggest names in sport.
"What a sad story," 17-times grand-slam winner Federer said after reaching the fourth round of the Australian Open on Saturday.
"I don't know what to say. It just really saddens me to see that someone did this for such a long time.
"Obviously he's hurt his sport in a big way, even though he helped it in the beginning. But now the burden they live under, all other sports maybe as well.
"I'm an active athlete right now and it's not fun times really to be in sports to a degree. I guess all I needed to see was the first few minutes of the interview and then I knew what was the deal, and the rest I don't really care.
"It's just very saddening this story, to be honest."
Serena Williams, who has won 15 grand slam titles, earlier admitted she had been "glued" to the televised confessional.
"I think as an athlete, as someone that works really, really hard since I was four or three, I think it's a sad day for all athletes in general," the American told reporters .
"But I think overall it's even more disappointing for the people that were adversely affected through everything. You can only just hope for the best for them.
"Unfortunately, I think a lot of people now look and are like, okay, if somebody that great (has been cheating) what about everyone else in every other sport?"
(Editing by Clare Fallon)