Australia believe they can beat the United States in the Davis Cup, despite retired captain Lleyton Hewitt replacing Nick Kyrgios. [Photo/Agencies]

Lleyton Hewitt says Australia's Davis Cup team is as strong as possible after the retired captain picked himself in the absence of Nick Kyrgios.

World number 27 Kyrgios has been ruled out of the World Group first-round tie against the United States in Melbourne after suffering from a back injury and a virus.

Hewitt has promoted Sam Groth to play alongside Bernard Tomic in the first singles rubbers and in the doubles.

But the 35-year-old, who called time on his career to considerable fanfare after the Australian Open in January, could yet select himself later in the tie, with the 2002 Wimbledon and 2001 US Open winner's retirement potentially set to last less than two months.

"Obviously with Nick, when we knew he had some issues, I tried to prepare as well as possible just to be another option," Hewitt said.

"I've been around enough Davis Cup ties now to know you've got to have back-ups and plans in place.

"I'm hitting the ball well and we've got as good a team as we can field going in and I'm confident we can still win."

US captain Jim Courier, who regularly works alongside Hewitt as a broadcaster during the Australian Open, was not expecting to see his counterpart named to compete.

"We've seen how much Lleyton was practising this week and with Nick's pretty conspicuous absence, but it's certainly not something coming down here that we were anticipating," Courier said.

Groth will play John Isner in the first singles rubber at the Kooyong Lawn Tennis Club on Friday, followed by Tomic against Jack Sock.