Britain's Joe Salisbury and American Rajeev Ram missed out on the ATP Finals doubles last four as they lost to Lukasz Kubot and Marcelo Melo.
Victory would have seen Salisbury and Ram reach the semi-finals, but they were beaten 6-7 (5-7) 6-4 10-7 by Poland's Kubot and Brazilian Melo.
Salisbury and Ram saved three match points before Ram sent a forehand into the net on the fourth.
Salisbury was the only Briton competing at the season-ending event in London.
In the singles event, Dominic Thiem of Austria and Italy's Matteo Berrettini play their final group match at the O2 Arena.
They are followed by Roger Federer and Novak Djokovic, with the winner of that match advancing to the semi-finals.
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