Ricciardo, 25, has impressed in his first season with the four-time constructors' champions, sitting third in the drivers' standings after two race wins.
Andretti, the 1978 world champion, was full of praise for the Australian, who he said was a "consistently perfect" overtaker.
"He's been fabulous. I'm sure that he has even surprised himself," Andretti told the F1 website.
"He has been consistently quick and doing it very well. I've been watching some of his overtaking and it's just consistently perfect - it's really aggressive but really well-calculated.
"He's showing a lot of class this year and his character is nice too - always with a smile. He keeps himself very cool and calm and he's doing his things really well.
"You could tell going in [to Red Bull] that he had nothing to lose because he was certainly not expected to outshine Sebastian [Vettel].
"But right from the beginning he took charge and he's still there. He's one of the special talents that you love to see now and then."
While Ricciardo has exceeded expectations, four-time defending champion Vettel has failed to live up to his in 2014.
Vettel is sixth in the standings with just two podium finishes this season.
Andretti backed Vettel to respond but believes the German would be disappointed with his campaign so far.
"He's a champion and that will show eventually, but I'm sure no-one is more disappointed than he is at this point," Andretti said.
"He's totally dedicated - he's one driver that I think is totally focused on his job; he has no other distractions and that's all he cares about.
"He'll get it done, I'm betting on that for sure. But I'm sure he's frustrated at the moment."