Jack Sock has kept his hopes of a semi-final place at the ATP Finals alive after coming from 4-2 down in the final-set tiebreak to overhaul Marin Cilic 5-7 6-2 7-6 (7-4) and claim a first group win.

The American – the first from his country to reach the event since Mardy Fish in 2011 – needed two and a half hours to beat the Wimbledon finalist from Croatia in a momentum-shifting match. He lost on Sunday in his opener to Roger Federer.

Six-time champion Federer and young gun Alexander Zverev were duelling for Becker group superiority later in a battle of undefeated players.

After Sock’s win, Federer can claim his 14th year-end semi-final from 15 appearances at the season wrap-up if he beats Zverev, 16 years his junior, in straight sets.

“It was a tough one for sure,” Sock said after defeating Cilic for the third time without a defeat in their series.

“It was an interesting day, we had a 4am fire alarm in our hotel also. I’m excited to excited to keep winning and keep my hopes alive this week.”

Cilic sunk to another defeat at the year-end showpiece, and now stands 1-7 for his career.

Sock added he played on instinct when he trailed in the tiebreaker.

“To keep myself right there like I did in the third set after being down a break, also in the tiebreaker, I can give myself a chance,” he said.

“There’s times, especially at the end of the year when you’re just thinking about your off-season, being able to go home the next day.

“But to fight through those, put yourself in position to do well in that tournament, now be here in London, to keep my hopes alive for this week as well by winning today.

“Definitely carries over, it gives you confidence when you’re in these matches, like losing the first set today, rebounding.”