London - Jack Sock found himself in the bizarre situation of being back at the ATP Finals as a doubles player on Monday - just a year after taking part in the singles event. 

The US player has plummeted to 106 in the singles rankings after a season to forget - in November last year he reached a career-high of eight following his win at the Paris Masters. 

The 26-year-old has endured a torrid time in 2018, winning just nine singles matches all year and losing 21. 

But he teamed up with Mike Bryan after Bryan's twin brother, Bob, suffered a season-ending hip injury in May and won the doubles titles at Wimbledon and the US Open. 

Coming into the ATP Finals at London's O2 Arena, Sock had won 35 out of 50 doubles matches and rose to a career-high ranking of number two in the world in September. 

The London season finale is contested by the eight singles players and eight doubles teams who have accumulated the most ranking points over the season and is in a round-robin format, with the best four players or teams reaching the knockout semi-finals stage. 

Bryan and Sock won their opening match on Monday against Lukasz Kubot and Marcelo Melo 6-3, 7-6 (7/5). 

"It kind of hit me this week, practising but I can't complain," said Sock, when asked if it felt strange to be back at the O2 playing doubles rather than singles. 

"It's so different being here from last year, top eight in the world in singles," he added. 

"Obviously I was on a high after Paris and it was an unreal finish to the year... hopefully I can be back here in singles next year, maybe singles and doubles, that would be fun."

Results on the second day of the ATP Finals at London's O2 Arena on Monday (x denotes seeding):

Singles

Guga Kuerten group

Alexander Zverev (GER x3) bt Marin Cilic (CRO x5) 7-6 (7/5), 7-6 (7/1)

Novak Djokovic (SRB x1) v John Isner (USA x8) 6-4, 6-3

Doubles

Knowles/Nestor group

Oliver Marach/Mate Pavic (AUT/CRO x1) bt Pierre-Hugues Herbert/Nicolas Mahut (FRA/FRA x8) 6-4, 7-6 (7/3)

Mike Bryan/Jack Sock (USA/USA x5) but Lukasz Kubot/Marcelo Melo (POL/BRA x3) 6-3, 7-6 (7/5)