MELBOURNE: New Australian Davis Cup captain Lleyton Hewitt spent his first day on the side of the court watching his team finish level with the United States after the opening singles in a World Group first-round match.

Whether he crosses the line and comes out of retirement to play for the home side over the next two days remains to be seen.

Bernard Tomic defeated American Jack Sock 7-6 (2), 6-3, 3-6, 6-4 on Friday after John Isner gave the US the lead with a 7-6 (2), 6-2, 6-2 win over Sam Groth on grass at Kooyong, the former home of the Australian Open.

Tomic broke Sock's serve in the ninth game of the fourth set, then clinched the match in the next game through four consecutive points. The match ended when Sock's backhand went long on a service return.

"Bernie stepped up when we needed him to," Hewitt said. "We knew the Americans were going to be tough."

On Saturday, Groth and John Peers were scheduled to play doubles against Bob and Mike Bryan, although that lineup can be changed on the morning of the match, as can the reverse singles on Sunday.

There is a chance that Hewitt, who came into the team for the ill Nick Kyrgios, could end a short-lived retirement to replace Groth in doubles. Hewitt retired from a 20-year career after he lost his second-round singles match at the Australian Open — less than two months ago.

Hewitt spent much of Friday's two singles matches pacing along the sidelines, clearly uncomfortable with not being able to take matters into his own hands on court. The 35-year-old is Australia's most successful Davis Cup player, having won 58 of the 78 singles and doubles matches he's played since his 1999 debut.

Isner took control of his match with Groth, who was elevated to singles duty because of Kyrgios' illness, when the first set went into the tiebreaker.

"I played a great tiebreaker and had a lot of momentum from that point forward," Isner said.

Defeat extended Groth's poor record in 2016. He's only managed one win, in the first round of the Australian Open, against seven losses.

Isner said his victory in 33 Celsius (91 Fahrenheit) conditions could cause more mental than physical damage to the Australian.

"Physically I think Sam will be fine," Isner said. "At this point for me and him, it's not a physical matter — it's probably a bit more mental.

"Any time you win a match in a situation like this you're on cloud nine, and you feel like you've got a lot of confidence. But on the flip-side, maybe not so much."

Groth, ranked 77th to Isner's 11, said he hoped to be playing doubles on Saturday.

"My body feels good and I feel like I've played good doubles in Davis Cup in the past and John and I have been playing well in practice," said Groth. "But it will be up to Lleyton. I will be doing everything I can to put my hand up to play."

U.S. captain Jim Courier said he felt his team had an edge with the Bryan brothers set to play on Saturday.

The Bryans have won 16 Grand Slam doubles titles and have a 23-4 record in Davis Cup. Australia's combination of Groth and rookie John Peers have only played one tournament together.

"Bob and Mike Bryan seem to be an advantage in the historical candor of doubles in Davis Cup," Courier said. "They've played awfully well and we feel confident in them. They've lost matches too, so we don't take anything for granted, but I love having them on our side."

The winner of the weekend matches will play the winner of Croatia and Belgium in the quarterfinals.