Nole is through to the semi-finals at the event for the eighth time.
Top seed Novak Djokovic improved to 2-0 in Group Guga Kuerten on Wednesday, outplaying Alexander Zverev 6-4, 6-1 at the Nitto ATP Finals.
Novak broke his young German rival three times en-route to a one hour, 16-minute victory.
Djokovic saved two break points at 4-4 and made a break in the following game to seal the opening set on his third set point.
The world no.1 raced to his second triumph at The O2 this week, breaking Zverev twice more, in the fourth and sixth games of the second set.
Since Cilic lost a set against Isner in their RR2 clash, Novak qualified regardless of score of Friday matches.
“I don’t think it was breathtaking tennis but a win is a win,” Novak said. “I played well mid-way through the second set and started to swing through the ball. I had not served that well but he made a lot of unforced errors which helped me win. He served very well in the first set, it was very hard to read his serve. I actually found it more difficult than in the first match against Isner.”
Match statistics: aces (1-9), double faults (3-2), winners (11-20), unforced errors (13-33), total points won (61-41).
Novak is bidding to lift a record-tying sixth trophy at the season finale.
The 31-year-old Serbian star plays his third Round Robin match on Friday at 20:00 local time (21:00 CET) against fifth seed Marin Cilic.
Group Guga Kuerten qualification scenarios after the completion of the 2nd round of the group stage:
1) Regardless of score, if N. DJOKOVIC defeats M. CILIC and A. ZVEREV defeats J. ISNER, then N. DJOKOVIC wins the group and A. ZVEREV qualifies 2nd.
2) Regardless of score, if M. CILIC defeats N. DJOKOVIC and A. ZVEREV defeats J. ISNER, then N. DJOKOVIC wins the group and A. ZVEREV qualifies 2nd.
3) Regardless of score, if M. CILIC defeats N. DJOKOVIC and J. ISNER defeats A. ZVEREV, then M. CILIC wins the group and N. DJOKOVIC qualifies 2nd.
4) If N. DJOKOVIC defeats M. CILIC and J. ISNER defeats A. ZVEREV in 2 sets, then N. DJOKOVIC wins the group and J. ISNER qualifies 2nd.
5) If N. DJOKOVIC defeats M. CILIC and J. ISNER defeats A. ZVEREV in 3 sets, then N. DJOKOVIC wins the group and A. ZVEREV qualifies 2nd.