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American Shelton, making his first ATP Finals appearance, looked set to force a third set but threw away a tiebreak that appeared all but won and Zverev came through 6-3 7-6(6) in their Bjorn Borg Group clash.

Shelton was overpowered by Zverev's serve in the opening set while the German, who won three service games to love, broke the American twice to take the opener in 28 minutes.

The American saved two break points to avoid going 3-1 down in the second, and hung in to force a tiebreak, where he went 4-0 ahead. Shelton was 6-3 up and serving for the set when Zverev made a stunning comeback with three successive breaks of serve.

The ATP Finals has a record prize pool of $15.5 million and offers 1,500 points. The tournament culminates with the final at the Inalpi Arena in Turin on 16 November.