The first part of the equation is done.
Djokovic struggled but still beat Milos Raonic in a 7-6 (6) 7-6 (5) thriller in London on Tuesday to collect that second victory and earn a place in the semifinals, where a meeting with Murray remains a possibility.
The other player to progress from group Ivan Lendl will be either Raonic -- defeated by Murray in the Wimbledon final -- or Dominic Thiem, the Austrian who downed Gael Monfils 6-3 1-6 6-4 earlier Tuesday at the O2 Arena. They face off Thursday.
France's Monfils, at 0-2, was eliminated when Djokovic edged Raonic.
This wasn't the almost perfect Djokovic of the first five months of 2016: In rallies, Raonic did much of the dictating and the world No. 2 twice relinquished a break lead in the second set.
But the Serb -- like Murray against Marin Cilic on Monday -- won the majority of the important points, no doubt aided by the psychological edge he held over Raonic. Raonic entered the clash 0-7 against the four-time defending champion, having won a single set.
"It was a very close match," Djokovic told reporters. "I think very few points separated us tonight. It really could have gone either way.
"All in all," he later added, "two tiebreaks against a big server is a great win and great confidence boost."
Questioned about his temper Sunday after smacking a ball in the direction of his coaches and not far from the crowd in his win against Thiem, Djokovic was mostly composed despite his inconsistent display -- although he did punish both feet with his racket when dropping serve at 2-1 in the second set.
The big-serving Canadian paid a hefty price for not capitalizing on three break points in Djokovic's first two service games.
And there was more misery for the world No. 4 in the opening tiebreak. He led 3-1 but was pegged back to 3-3 after failing to put away a forehand volley. A tactical error followed with Djokovic leading 4-3: Raonic didn't challenge a Djokovic forehand down the line called in -- replays showed the ball to be wide.
Minutes earlier he challenged an almost identical shot -- and was proved correct.
Djokovic manufactured a first set point with a delicate backhand lob, only to see Raonic save it with a blistering forehand as Djokovic went passive. But on a second set point, Raonic double faulted long.
When Djokovic lost his serve in the fourth game of the second, he broke straight back and even Raonic must have thought that was that. Djokovic, though, conceded serve from 30-0 at 4-3 and Raonic -- ninth last in return games won among more than 50 players listed on the ATP's website -- broke back again.
"I'm not very pleased to drop my serve twice against Milos, especially the second time," said Djokovic. "I was 4-3, 30-love, then just four pretty bad unforced errors.
"Credit to him for really hanging in there, putting pressure, being aggressive, especially from the forehand. But I think I should have done better there."
Djokovic then escaped from 15-30 at 4-5 and Raonic wasted a set point at 5-6, sending a forehand down the line into the net. He led 2-0 in the second tiebreak and by another mini-break at 4-3 but Djokovic authored the shot of the match at 4-5, a stunning half volley.
The 12-time grand slam winner hit an ace on the next point, one of his 11 winners -- his unforced error tally was 22 -- compared to Raonic's 42. Raonic's 27th unforced error officially sealed the contest.
"I could have won it," Raonic told reporters. "I gave myself some chances there. He played well in most of those chances. It is what it is. Sometimes us tennis players, we struggle to forget. But that's the number one thing I've got to do right now."
Murray (1-0) returns to action Wednesday in the John McEnroe group against Kei Nishikori (1-0), the Japanese baseliner who upset the Scot at the US Open in September. US Open winner Stan Wawrinka (0-1) battles former US Open champion Cilic (0-1).