Watch: An incredible act of sportsmanship

Lleyton Hewitt reacts after Jack Sock's display of sportsmanship.

If opponents keep treating Lleyton Hewitt like this, he might be able to go on a serious run at this year's Australian Open.

The soon-to-be retired Aussie champ was floored by an incredible act of sportsmanship from American youngster Jack Sock at the Hopman Cup on Tuesday night.

Serving down 4-5 in the first set of their singles match, Hewitt had his first serve at 30-0 called wide. But as Hewitt returned to the service line and prepared to send down his second serve, Sock surprised everyone in attendance by saying to his opponent: "That was in if you want to challenge it."

Hewitt could barely believe his ears and appeared uncertain as to whether his opponent was telling the truth, or perhaps trying to trick him into wasting a challenge.

But he eventually took Sock at his word and called for the chair umpire to review the call. When Hawkeye revealed the ball was catching the line, the crowd roared.

Sock was telling the truth.

Hewitt rediscovered his winning mojo later in the match to further bolster his preparations for his Australian Open swansong.

He helped lift Australia Gold to a 3-0 win over the US by eventually defeating the world No. 26 7-5 6-4.

It came just two days after he put up a good fight in a 6-3 6-7(7-1) 6-4 loss to world No.41 Jiri Vesely.

Hewitt will retire after this month's Australian Open. And he's on track to go out with a bang.

"I said I was going to be better from that first match," Hewitt said after his singles win over Sock.

"I felt like the second and third sets (of that first match) I was starting to get the feel of the tempo of playing against these big guys again.

"The first two matches I've played pretty big servers - one lefty, one righty. I felt like for the most part I handled it pretty well (against Sock).

"Jack's a tough player. A massive forehand, big serve. He slices pretty well.

"I felt like I had to try to keep enough balls in, but I couldn't go too defensive and let him dictate.

"I felt like I stepped it up from two nights ago."

- with AAP