Cilic was beaten in three sets by Alexander Zverev on Sunday in match he felt he had chances to win, especially after taking a 3-1 lead in the deciding set.
However, the 20-year-old German roared back to win five of the next six games and inflict Cilic’s sixth ATP World Tour Finals defeat in seven appearances.
Sock meanwhile could not even take the comfort of a set from his opening encounter with six-time champion Roger Federer, who eased through the gears to secure a 6-4, 7-6 victory.
The American qualifed for the competition at the last possible moment, capitalising on injuries to Federer and Rafael Nadal to win the Paris Masters, the biggest win of his career to date.
Cilic vs Sock LIVE
(* denotes next to serve)
*Cilic 2-2 Sock
Sock brings up 30-0 with a beautiful volleyed lob over the huge Cilic, who battles back to parity. But at game point Sock, he shows one of his weaknesses, charging net and frame a relatively simple backhand volley.
Cilic 2-1 Sock*
A much better game from Cilic, and that forehand is starrting to fire. Five winners in total now, including one from well behind the baseline. Sock has already got a Murray-esque mumble on.
*Cilic 1-1 Sock
Frustration for Cilic as he gets into the game at 30-30 only to produce his fifth unforced error of the match. But he works his way into the next rally and forces Sock wide for deuce. A very poor slice backhand from the America barely reaches the net to bring up break point Ciilic…and while Sock does run around the forehand he loves so much, it flies only just long and Cilic breaks back immediately.
Cilic 0-1 Sock*
Cilic’s serve will of course play a big role in this match and Sock shows him what he thinks of the second serve, battering a forehand winner back at a cool 100mph. A wayward, spinny Cilic forehand brings up an early break point for the American but Cilic hits a beautiful backhand winner to save it.
Another chance presents itself after the Croatian fails to pick up a half-volley this time Sock breaks as Cilic hits the tape with a forehand. First blood to the American.
2.15pm: The players have nearly finished knocking up which gives us a quick chance to look at their head-to-head records.
Both of their previous meetings came on American hard courts and the home man triumphed both times: Sock beat Cilic in a Davis Cup five-setter from 2-0 down before knocking him out of the 2016 US Open in more routine fashion.
Cilic to serve…
2.05pm: Right. The doubles (or Barney Rubbles as one Express correspondent amusingly calls them) are done in a relatively timely fashion and the arena has been plunged into darkness for the dramatic entrances of the players. One thing that does do is hide the empty seats of which there are still plenty.
1.45pm: Woken up yet? That was a late one last night as Rafael Nadal ran himself into the ground in a 157-minute epic against David Goffin before admitting defeat to a knee injury in the immediate aftermath.
Without the Spaniard, it opens up the draw for the likes of Cilic and Sock – although losing for a second time this evening would make qualifying for Saturday’s semi-finals almost impossible.
The doubles clash between Henri Kontinen & John Peers and Jean-Julien Rojer & Horia Tecau appears to be running long – they are about to go into a champions tie-break to decide the match – so we might be starting closer to quarter past two than 2pm.