A bout of acute sportsmanship has broken out at the Hopman Cup in Perth, with Lleyton Hewitt bowled over by an incredible gesture from American rival Jack Sock in their tennis clash on Tuesday night.

The soon-to-be-retiring Australian champion, who trailed 4-5 in the second set, was serving against Sock 0-30 down when his first service was called out by the linesman.

Incredibly, Sock immediately signalled to the chair umpire, saying to Hewitt with a smile: "That was in if you want to challenge it."

Seemingly baffled, Hewitt could not believe what was taking place before eventually succumbing to the gesture and asking for a review.

After a nervy wait - an 'out' call would have made Sock look a bit of a fool - the ball was shown to be in, prompting a huge roar from the crowd.

It helped Hewitt rediscover his form in the match, going on to win 7-5, 6-4, having previously beaten Jiri Vesely in three sets.

"I said I was going to be better from that first match," Hewitt said after beating 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.

"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."

Hewitt will retire after this month's Australian Open.