Govia Thameslink (GTR) and the RMT rail union will meet for a third day at the conciliation service Acas from midday.
The two sides are in a long-running dispute about GTR's plans for drivers to open and close carriage doors instead of conductors.
The walkout is due to begin next Monday, with 40% of normal services cancelled.
An Acas spokesperson said: "Acas talks have adjourned for the day and will resume tomorrow."
GTR said it could not make any further comment.
It had previously said it needed a resolution on Thursday in time to implement an emergency timetable next week.
Lines with no rail service during the strike are expected to include:
Talks to avoid a five-day strike on Southern trains have been adjourned until Friday.