Tomorrow's matches are going to be very interesting.
The permutations are absolutely mind-blowing but Roger Federer's best chances of qualifying are with a win.
He takes on Kevin Anderson in the evening session, following Kei Nishikori's clash with Dominic Thiem.
You can follow live text commentary on our website, on BBC Two and by listening to Radio 5 live.
Here are the next matches in group Guga Kuerten, which will take place on Friday.
Marin Cilic on Sky Sports: "It was very difficult. The first set was extremely tough. John [Isner] played a great tie-break and I had to stay focused.
"I served really well and I am really pleased with the win. I felt that I had that momentum towards the end of the second set. I felt the ball well and got some good points on Isner's serves.
"He hit a few double-faults on the break points - that helped - but I stayed patient and I managed to break back and play some good tennis towards the end.
"There was a little bit of luck involved. I really enjoyed the atmosphere tonight. It is so much fun and I enjoy playing here. I am looking forward to the next match in the group."
On facing Novak Djokovic next: "It was a tough match in Paris. He is playing really good tennis but I have to keep playing well, serve well and hopefully I can have another great match."
World number one Novak Djokovic becomes the first player to qualify for the semi-finals of this year's competition thanks to Cilic's third-set victory.
He will face Cilic in his final group game but will be through no matter the result.Copyright: Getty Images/BBC Sport
John Isner's tournament is not yet over as he could still qualify despite losing his first two matches.
Marin Cilic has helped his case but there is one man already through...
A looping second serve is skewed wide by Isner and Cilic has his second win at the ATP Finals!
A wide serve and a thumping forehand winner takes Cilic to 15-0.
Another big serve down the middle isn't returned over the net by Isner. 30-0.
And Cilic moves 40-0 clear with a one-two punch on the forehand.
Three match points...
Over to Cilic.
Will he crumble or will he power through? The Croatian will serve for the set and the match.
A forehand bounces centimeters wide of the line - as shown on the review - and Isner smashes his next forehand into the net as Cilic moves ahead on 30-0.
Isner responds with a big serve and Cilic flops a forehand down the line to draw things level at 30-30.
A huge serve from Isner takes it to 40-30. And he rounds it off with an ace.
Isner's early return goes inches wide of the line as Cilic opens the game at 15-0. But his next backhand slams into the net, followed by a forehand... 30-15 to Isner.
He responds with a low, wide forehand which Isner can only lob out of play. 30-30. Another wide backhand from Isner takes Cilic up to 40-30 and the Croatian fires a terrific serve to hold.
One game away from victory.
Two more service games for Cilic. If he holds both, he will pick up just his second ATP Finals victory in 11 matches and his first since 2016.
Still has a lot of work to do though.Copyright: Reuters
Isner moves up to 30-0 and smashes a forehand winner into the open court to make it 40-0 in no time.
The American overcooks his backhand but rounds the game off with a forehand which Cilic blasts wide.
Cilic's backhand slams into the net. But Isner's next return is long. 15-15.
The Croatian pumps his fist as he slams an ace down the middle. 30-15. And follows it up with a huge second serve in the same place. 40-15.
He completes the game with a wide serve which is returned wide.Copyright: AF
Isner moves up to 30-0 but Cilic is chasing everything on the serve. The Croatian's forehand is just short of the net though and Isner moves up to 40-0.
Isner then fires a serve on the line - confirmed by a challenge. Nice hold!
That's some game... how will Isner respond?
An big serve down the hammer is a good way to start the game. 15-0. Cilic follows it up with a forehand winner down the line. 30-0.
And he comes out on top in the rally as Isner's backhand bounces inches over the baseline. 40-0.
He wins the game to love as Isner's backhand slams into the net.
The pressure is taking its toll in the service games. Can Cilic answer his critics with a cool and calm response here?
One saved. Isner serves wide and Cilic's return doesn't make it past the net end.
But the Croatian delivers another superb return and drops a volley inches over the top of the net! He's back in the lead.