Djokovic needs to win all three of his Group A matches to ensure he will hold off Roger Federer in the race for first place.
He secured the second of those victories with a ruthless 65-minute rout at London's O2 Arena.
The reigning Wimbledon champion will be guaranteed top spot if he beats Tomas Berdych on Friday in a match he also has to win to be certain of making the last four at the prestigious season-ending event.
Wawrinka's loss means the Swiss world number four needs to defeat US Open winner Marin Cilic, thrashed 6-3, 6-1 by Berdych earlier on Wednesday, to have a chance of making the last four.
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