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Fittingly the London era ended as it began with a Russian winner after Nikolay Davydenko took the 2009 title.
Rafael Nadal's dream of a first ATP Finals title and Novak Djokovic's quest for a record-equalling sixth vanished in defeats by Daniil Medvedev and Dominic Thiem in two seismic semifinals.
The 27-year-old Austrian appeared to be sliding to defeat when he trailed 0-4 in the third-set tie-break but stormed back to claim a 7-5, 6-7(10), 7-6(5) victory in just under three hours.
World No. 1 Novak Djokovic beat Alexander Zverev 6-3, 7-6(4) to seal a last-four spot.
The Spaniard beat defending champion Stefanos Tsitsipas 6-4, 4-6, 6-2 in the last round-robin match in the London Group.
The Russian was ice cool as he closed out victory to make it two wins from two in the group and guarantee a semifinal berth.
The world No. 3 struck 37 clean winners to claim his second round-robin victory and reigning champion Stefanos Tsitsipas's narrow defeat of Andrey Rublev later guaranteed the Austrian a semifinal spot with a match to spare.
Medvedev's 6-3, 6-4 win over Zverev put him joint top of the Tokyo Group alongside world No. 1 Djokovic, who put on a masterclass to trounce debutant Schwartzman 6-3, 6-2.
Austrian Dominic Thiem got his campaign off to a fine start with a 7-6(5), 4-6, 6-3 victory over Stefanos Tsitsipas.
Alexander Zverev and Stefanos Tsitsipas both crashed to defeat in their first matches of the year Friday as day one of the inaugural ATP Cup sprung some big upsets.