Tomas Berdych may yet progress to the last four of the ATP World Tour Finals after victory over Andy Roddick on Wednesday.
Berdych defeated Roddick 7-5 6-3 in their Group A match at London's O2 Arena, with the Czech improving to a 1-1 record after an opening defeat to world No.3 Novak Djokovic.
Roddick, the lowest ranked of the eight players to contest the season-ending tournament, opened with defeat to world No.1 Rafael Nadal.
The 28-year-old American now looks likely to bow out at the group stages after two defeats from two matches played.
Berdych saved two set points when 4-5 down in the opening set before breaking Roddick for the first time in the following game.
The 25-year-old world number six then held his serve and went on to see out the set.
Roddick lost serve and his cool at 2-2 in the second, throwing his racquet after an easy smash fell narrowly wide.
He challenged the call but was unsuccessful, with the missed shot proving to be a turning point as Berdych grew in confidence while his opponent's concentration deteriorated.
Clearly flustered, Roddick quickly unravelled and, finding himself 3-5 down, handed victory to Berdych when a tame forehand on match point could only find the net.
The 2003 US Open winner will be eliminated if Djokovic defeats Nadal in the second Group A match later on Wednesday.