Sock blasted a ball into the crowd in frustration, but channeled his anger to break in the first game of the second set and was clearly enjoying himself, moving around the court nimbly as he levelled the match in dominant fashion.
Cilic broke for a 2-0 lead in the deciding set, but Sock displayed the fighting spirit that carried him to an unlikely Masters victory in Paris last month, breaking back in the fifth game and force a tiebreak.
The match then settled back on serve and went to a tie-break.
Cilic looked set to come out on top when he claimed the first mini-break of the tie-break, but Sock showed incredible athleticism to dart in and guide away a victor after a Cilic shot struck the net cord and he won five points in a row to seal a hard-fought victory.
"It's been an interesting morning so far". We had a 4am fire alarm in our hotel also.
"I expect him to play near his best", said. It's an incredible atmosphere, you make me feel like home.
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Sock lost his debut match at the O2 Arena to Roger Federer, but he is now 1-1 in his round-robin group.
"I'm excited to excited to keep winning and keep my hopes alive this week".
The top two players from each of the two groups of four will progress to the semi-finals.
Rafael Nadal's dramatic withdrawal from the ATP Finals in London leaves Roger Federer as the only megastar standing at a tournament now looking seriously short of star power.
The triumph in 2 hours 11 minutes puts the 19-times grand slam champion in Saturday's semifinals - the 14th time he has made the last four in 15 appearances - with the tournament still only three days old.