Kei Nishikori's return to the top echelons of the men's game in 2018 has been a remarkable story. Nadal cited an abdominal injury to close his season early - he hasn't played since the US Open. "I couldn't really see that happening at the beginning of the year".
If he wins the tournament, which starts on Sunday, he would draw level with Roger Federer with six wins in total.
On facing Djokovic first up, he said: "I'm up against it for sure. I don't have much to lose, so I'll play loose and free".
He sported a poppy, the symbol the British wear at this time of year to honour those who died in the world wars, and exalted in being a tourist in London, though his hotel room opposite the Houses of Parliament offers a view onto scaffolding and cladding round Big Ben, rather than the clock itself.
Cilic, Anderson, Nishikori and Thiem are all Grand Slam finalists.
This tournament opener is a hard match to call as although Anderson has mostly had the upper hand in this particular rivalry, it is the Austrian who has won both their meetings in 2018. "The 6'8" Anderson had battled to victory after being two sets and match points down (in third set) against Federer in the Wimbledon quarter-finals. Also in his first ever ATP Finals is American veteran John Isner. While Anderson had a breakthrough last season, making it to the US Open finals, the 33-year-old Isner has taken a little longer.
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Djokovic is set to end the season at the top spot after Nadal made a decision to withdraw from the London event.
When is Kevin Anderson vs Dominic Thiem on and what time does it start? . "I've learned that I need to be 100 per cent from the very first ball".
Cilic, who was runner-up at the Australian Open, has qualified for the fourth time he has only one win (over Nishikori in 2016) to show so far.
The 31-year-old, who slipped to 22nd in the world in May, admitted that after his surgery it looked improbable that he would finish the year as number one.
Nishikori returned to action in January after a five-month absence due to a right wrist injury but fell to number 39 in the ATP rankings in April, his lowest position since 2011.
The two men, with 34 Grand Slams between them, are the twin titans at the season-ending event at London's O2 arena.