Novak Djokovic and Daniil Medvedev, the two top-ranked players in the world, are the main attractions of the Davis Cup which attempts to re-establish itself this week after the cancellation of the 2020 event.

The Davis Cup, which began life back in 1900 as a bilateral contest between Britain and the United States, has had numerous makeovers in the past, most recently in 2019 when it was reshaped to resemble a world championships.

This year's event will be played by 18 teams, initially in six groups, over 11 days across three cities.

The group stages, which start on Thursday, will be played in Madrid, Turin and Innsbruck where the surge in coronavirus cases and the subsequent lockdown in Austria means that matches will be played once again in an empty arena.

The semi-finals and December 5 final will all be played in the Spanish capital, reward for the team that won the title two years ago.

Back then Spain could call on Rafael Nadal.

However, this time around the 35-year-old is out of action, recovering from a foot injury that restricted him to a single tournament since losing to Djokovic in the semi-finals of the French Open.

The 20-time Major winner is anticipating a return to the courts in Abu Dhabi next month.

The holders face a tough battle with Medvedev's Russian team on Sunday if they are to progress from their group to the quarter-finals.

"It's true that we had a big advantage," said Spanish captain Sergi Bruguera.

"If you have Nadal in your team you are straight in as favourites because he is the best player in the history of the Davis Cup.

"But he's not here, we cannot think about this."

Spain still have three players, including Pablo Carreno Busta, who were a part of the 2019 winning team and in 18-year-old Carlos Alcaraz they have one of the most exciting young players on the tour.

Russian hopes in Group A, which also contains Ecuador, rest on an impressive all-round team.

World number two Medvedev, whose season peaked with a three-set annihilation of Djokovic in the US Open final in September, is backed up by two others in the top 20 -- Andrey Rublev (5) and Aslan Karatsev (18) -- as well as Olympic silver medallist Karen Khachanov (29).

"There are lots of strong teams -- Serbia with Novak, Spain, of course, in our group, Italy even without Matteo Berrettini and the USA. But we believe in ourselves," said Khachanov.

Serbia's hopes of replicating their 2010 success will rest largely on the shoulders of Djokovic.

However, the world number one looked weary at the ATP Finals in Milan last week when he lost to Alexander Zverev in the semi-finals.

The German has chosen not to play in the Davis Cup and he is joined on the sidelines by a host of other big names as 10 of the world's top 15 are absent.

Berrettini (7) is still suffering from the abdominal injury that ruled him out of the ATP Finals.

Dominic Thiem (15) is missing for Austria as he continues to recuperate from an injury that wiped the latter part of his season while 2019 finalists Canada will be without the injured Felix Auger-Aliassime (11) as well as Denis Shapovalov (14) who is resting.

And then there are those whose teams did not qualify -- Stefanos Tsitsipas (4, Greece), Casper Ruud (8, Norway), Hubert Hurkacz (9, Poland) and Diego Schwartzman (13, Argentina).

The 2021 Davis Cup begins on Thursday with Canada taking on Sweden in Group B in Madrid, France up against the Czech Republic in Group C in Innsbruck and Croatia battling it out with 28-time winners Australia in Group D in Turin.