The ATP has announced the opening of a tender process for the staging of the ATP Finals beyond 2020.
The prestigious season-ending event has been staged at the O2 Arena in London since 2009 and will be for at least the next three editions.
London could continue to host the best eight male singles players and doubles teams in the world after 2021, but the governing body has encouraged rival bidders to express an interest.
ATP executive chairman and president Chris Kermode said: "Our season-ending tournament has been a remarkable success story since it moved to London in 2009, and we look forward to continuing the tournament's growth over the next three years at The O2 through to 2020.
"The tournament’s popularity and stature have reached unprecedented heights in London, which has attracted significant interest from potential future host cities over the years."
"We feel the time is right to undertake this international tender process as we look to secure the event's future beyond 2020."
"The O2 has set a very high bar, and we expect that staying in London beyond 2020 will also be among the options for consideration as we go through this process."