The Serbian five-time champion prevailed 7-6, 7-6 against the American, who was making his Finals debut, to move within one victory of equalling Swiss Roger Federer's record of six titles at the season finale.

A spirited display from Fritz saw him recover a break deficit in the first set and surge to a 5-3 lead in the second at the Pala Alpitour.

Despite struggling for consistency, Djokovic managed to pull through in both tie-breaks to seal victory in 1 hour 54min.

"I had to fight to survive," said Djokovic.

"I didn't feel very reactive today or very comfortable.

"I knew coming into today's match from yesterday's gruelling battle against [Daniil] Medvedev I knew it would take me some time to adjust and find the dynamic movement I need against Fritz, who is one of the best servers on the Tour."

Casper Ruud's dominant 6-2, 6-4 victory over Andrey Rublev, who was playing as a neutral, saw him reach new heights.

The world number four was clinical in the key moments as he converted all four break points to cruise to a 69-minute triumph.

The only blip for him came at 5-2 in the second set, when he was serving for the match but was broken.

The 23-year-old made no mistake at the next time of asking, winning the game to love.

Ruud will be the first Scandinavian to contest an ATP Finals championship match since Sweden's Stefan Edberg in 1990.

Should he defeat Djokovic and clinch the title, Ruud will pass Spain's Rafael Nadal to end the year as world number two in the ATP Rankings.