Cilic, who won the 2014 U.S. Open, broke the American twice to win the first set but found himself adrift in the second as Sock levelled the round-robin match with two breaks of his own.
Both players held their serve for the remainder of the opening set until the former US Open champion secured a decisive break in the 12th game of the match to take the initiative.
"It's been an interesting morning so far".
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Despite having been forced out of bed in the early hours of Tuesday morning by a fire alarm at his hotel, Sock made a fast start - breaking in the opening game, only to hand it straight back to his opponent.
"I expect him to play near his best", said. "Thank you for making it such a special night", he said.
The Spaniard is the only one of the "Big Four" never to have won the ATP Finals and has pulled out a number of times previously before the event, which is played on a fast indoor surface rather than his favoured clay.
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Ultimately, the more experienced Croat Cilic, who has a poor record in the end-of-season finals, proved just too erratic, mixing his 35 outright winners with 37 unforced errors.
Federer is through to the semi-final stage after beating Alexander Zverev in three sets, 7-6, 5-7, 6-1.
Rafael Nadal's dramatic withdrawal leaves Federer as the only genuine star left in London and is the clear favourite to lift his seventh year-end title at the grand old age of 36.
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Sock defied the odds to qualify for the season-ending tournament for the first time and boosted his chances of reaching the semi-finals with a 5-7 6-2 7-6 (7-4) victory over the fifth seed in Group Boris Becker.
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