Djokovic. No. 2 in the world rankings, dropped his serve just once on the day of a 6-3, 3-6, 6-3 win over del Potro, who is ranked fifth. Djokovic faced five break points, including two in the third set. He took 75 percent of the points on serve in the third set.
He improved to 2-0 in Group B play with a match against ninth-ranked Reach Gasquet, who is 0-2 in the group after a 6-4, 6-3 loss to Roger Federer earlier Thursday, yet to be played. With Djokovic locking up one Group B semifinal slot, del Potro will go against Federer, No. 7 in the rankings, for the other on Saturday. Djokovic beat Federer in last year Tour Finals title match.
Djokovic used a sixth-game break to claim the first set but del Potro broke through in the sixth game of the second set and extended the match. The telling break in the third set also came in the sixth game but by Djokovic at love and aided by several del Potro errors.
Djokovic then lost one point in his last two games serving.
Federer scored a break in the third game for an early advantage in his Thursday match. Gasquet got back to even with a break in the eighth game but Federer put together a strong run and won 16 of the next 19 points.
The spurt sealed the first set and put Federer ahead 2-0 in the second. Gasquet had four break chances through the rest of the set but was never able to convert. Federer put the match away with his fourth break of the day.
That Federer needed six chances at match point to end the battle was indicative of the day. He survived despite having more than two dozen unforced errors.
Group A wraps up its play Friday with No. 3-ranked David Ferrer (0-2) going against No. 8 Stanislas Wawrinka (1-1) in the day match while No. 1 Rafael Nadal (2-0) meets fifth-ranked Tomas Berdych in the evening session. Nadal has secured a place in the semifinals and Ferrer has been eliminated.