Andy Murray's route to securing the year-end world number ranking looks fraught with danger after the top seed was grouped with Stan Wawrinka, Kei Nishikori and Marin Cilic for next week's ATP World Tour Finals in London.
Murray, who reached the number one ranking for the first time on Monday to end Novak Djokovic's 122-week reign, will begin his group round-robin matches on Monday against in-form Croatian Cilic.
Djokovic, whose form has dipped since he completed his career grand slam by winning the French Open for the first time in June, was placed in an easier-looking group with tournament debutants Gael Monfils and Dominic Thiem and Canada's Wimbledon runner-up Milos Raonic.
He has a combined 23-0 career record against that trio, leading Monfils 13-0, Raonic 7-0 and Thiem 3-0.
The 12-times grand slam-winning Serb will open on Sunday against world number nine Thiem.
Murray usurped Djokovic after winning the Paris Masters against John Isner on Sunday, although he was already assured of becoming the 26th player to top the ATP rankings when Raonic pulled out of their semi-final on Saturday.
Djokovic lost to Cilic in Paris at the quarter-final stage.
Murray has strung together 18 consecutive wins and has won his last four tournaments since losing in the U.S. Open quarter-finals to Japan's Nishikori.
Cilic beat Murray in the Cincinnati final, shortly before the U.S. Open, while Swiss Wawrinka, the U.S. Open champion, has beaten Murray seven times in 16 meetings, including at the Tour Finals last year when he won in straight sets.
Murray holds a 405-point lead over Djokovic but his record at London's O2 Arena has been unimpressive.
In seven previous appearances he has never reached the final, losing three times at the semi-final stage.
Defending champion Djokovic has won the title for the past four years, losing only one round-robin match in that time.
With 1,500 points available for an undefeated champion next week, the tournament could produce a thrilling climax with Murray and Djokovic, who have never met at the O2, facing off in the final for the year-end number one ranking.
"It's nice to play in London ranked one, but once you are on the court you are not thinking about your ranking," Murray told the ATP website.
"It's the top eight players and every match will be tough."
Full draw: (world ranking in brackets)
John McEnroe Group
Andy Murray (Britain) (1)
Stan Wawrinka (Switzerland) (3)
Kei Nishikori (Japan) (5)
Marin Cilic (Croatia) (7)
Ivan Lendl Group
Novak Djokovic (Serbia) (2)
Milos Raonic (Canada) (4)
Gael Monfils (France) (6)
Dominic Thiem (Austria) (9)
*Top two players from each group contest semi-finals