"I've never been to Perth - I've never played the Hopman Cup, so for me it's going to be a wonderful experience," Williams said.
"I've watched it literally every year on TV so now I get to play, I'm very excited."
World number 10 Isner won the title in 2011 with Bethanie Mattek-Sands, who stands at 5ft 6in (1.68 m)-tall to his 6ft 9in (2.06m). Williams is 6ft 1in (1.85m).
"My previous two partners have been very, very small," said Isner.
"But Venus is a little bit taller so I'm not going to look like such a giant out there. She's obviously one of the greatest players ever so I think our team has a good shot to do well."