Novak Djokovic fought past Alexander Zverev but it wasn't enough to stave off defeat for Serbia in the ATP Cup Friday, with Germany making the semi-finals alongside a Spanish team missing injured Rafael Nadal.

The world number one, who is ramping up preparations to defend his Australian Open crown, was tested in his singles rubber before outlasting Zverev 6-7 (3/7), 6-2, 7-5.

But the German got revenge in the deciding doubles, teaming with Jan-Lennard Struff to edge Djokovic and Nikola Cacic 7-6 (7/4), 5-7, 10-7 and ensure they made a last four clash against Daniil Medvedev's Russia.

Struff earlier beat Dusan Lajovic in the other singles tie.

"It was so difficult to play against (Zverev) today. Missiles from the other side of the court, both first and second serve," said Djokovic, who led his country to the inaugural title last year.

"He was going for it, not too many double faults. You've got to give him credit for being courageous and trying his best."

Nadal missing again

Meanwhile, Nadal was once again missing from the Spanish team as they lost 2-1 to Greece. 

He watched from the team box as Pablo Carreno Busta and Roberto Bautista Agut led the challenge against Michail Pervolarakis and Stefanos Tsitsipas.

Carreno Busta, ranked 16th, had few problems in thrashing Pervolarakis 6-3, 6-4 to guarantee their spot in the playoff rounds against Italy.

Australia was already eliminated from their group heading into Friday's match, while Greece needed to win the tie 3-0 to advance.

"With this victory, we are into the semi-finals, so it was very important to us," Carreno Busta said. "I tried to be focussed on myself, to try to play my best game."

Bautista Agut couldn't match the exploits, crashing 7-5, 7-5 to world number six Tsitsipas, who boasts two wins from two in a promising start to his season.

"I think I played very solid throughout the whole match and showed aggression," said the Greek. "My forehand was on fire, my serve was working really well for me."

The Dominic Thiem-led Austria meet France later Friday with the final group tie between Argentina and Japan on Saturday.

Eyes on the Open

The results mean 17-time Grand Slam champion Djokovic has four tough matches under his belt -- two singles and two doubles -- as he targets a record-extending ninth Australian Open title.

In contrast, Nadal hasn't played a competitive game since the semi-finals of the ATP Finals in London on November 21.

His only appearance since was an exhibition against Thiem in Adelaide last week, with a back injury casting a shadow over his hopes of a record-setting 21st Grand Slam title in Melbourne.

He pulled out of Spain's opening ATP Cup tie at short notice as well as missing the Greece game on Friday.

"I'm not well enough to play. I'm improving bit by bit, but I'm not ready to play a match with maximum intensity," Nadal told Spanish broadcaster Vamos.

The Australian Open gets underway on Monday with Nadal, the 2009 champion, looking to edge ahead of Roger Federer who also has 20 Slam titles to his name. 

The injured Swiss great is not playing and will only return to the tour in Doha in March.