The year's eight best singles players have assembled at the O2 Arena ahead of the ATP Finals beginning on Sunday.

The tournament is being held in London for the 11th and penultimate time, with Alexander Zverev defending his title.

Here, the PA news agency picks out five things to look out for.

The battle for number one

The generational shift in men's tennis has been akin to the changing of the guard outside Buckingham Palace being performed by snails. Atop the rankings still sit Nadal, Djokovic and Federer, with a combined age of 103. But the five players joining them in London are all aged 26 and under, with only Dominic Thiem older than 23. Zverev struck a blow for the young guns by winning at the O2 last year and this is a chance to shape the narrative heading into 2020.

Zverev and Tsitsipas resurgent

For a long time it appeared Zverev would not even make it to the start line in defence of his title. Off-court issues and problems with his serve have contributed to a difficult season but the German has been much improved recently, including beating Federer in Shanghai. After his stellar start to the season, Tsitsipas hit the buffers over the summer but also found his form again to qualify for a debut appearance at the O2, where he is sure to be a big draw.

Man of the moment Medvedev

The Russian had been rather overshadowed by other rising stars like Zverev but he has come of age spectacularly over the second half of the season. From July to October he reached a remarkable six successive finals, including at the US Open where he made a dramatic transformation from pantomime villain to hero with a stirring comeback against Nadal. His mentality and skills both in defence and attack have led many pundits to label him the next new grand slam champion and he will have high hopes for his O2 debut.

Salisbury flies the flag

The man to maintain Britain's record of always having a home qualifier for the O2 is someone even tennis fans may struggle to recognise on the street. But that is certainly not to denigrate the achievements of 27-year-old Londoner Joe Salisbury, who will make his debut in the doubles alongside American Rajeev Ram. The pair have won two titles this season and and are in fine form. No British player has ever won the doubles title at the ATP Finals.