Nadal started the day with a 6-3, 6-2 victory over third-ranked David Ferrer and Djokovic followed with a 6-4, 6-7 (2-7), 6-2 win over No. 7 Roger Federer that extended his personal winning streak to 18 matches.
Djokovic allowed Federer one break opportunity in the first set but Federer missed the chance and Djokovic compounded the issue with an immediate break that was worth the first set. Federer had two second-set breaks but lost the advantage both times. However, he ran off the last five points of the tiebreaker to even the match.
Djokovic broke Federer at love to open the third set and survived a break point in a 12-point game to go up 2-0. Federer held at love but Djokovic claimed the next three games, including a break -- his fifth of the match -- to frustrate Federer in three sets for the second time in four days.
The result left Djokovic tied with fifth-ranked Juan Matrin del Potro for the Group B lead with 1-0 records. Del Potro won Monday's Group B match 6-7 (4-7), 6-3, 7-5 over ninth-ranked Richard Gasquet.
Nadal started the tournament needing two match victories to sew up his third year-end No. 1 ranking and got the first one against Ferrer in a Group A match.
Ferrer snapped a nine-match losing streak against Nadal with a win Saturday in the Paris Masters semifinals but Nadal on Tuesday quickly made sure Ferrer didn't get another win.
Nadal took 56 percent of the points off Ferrer's serve and finished off six of his seven break chances. He had only one break in the loss last Saturday.
Group A takes the court alone Wednesday with Nadal and Wawrinka playing in the early match and Ferrer meeting Berdych late in a virtual elimination match.