Three-time defending champion Magnus Carlsen was neck and neck with American Fabiano Caruana after eking out a draw in their sixth world championship game on Friday.
"I made many mistakes," the 27-year-old Carlsen -- nicknamed the "Thor of Chess" -- said after a gruelling six-and-a-half hour match with 80 moves.
"I just got way too casual," the Norwegian added.
Carlsen is seeking to cement his reputation as one of history's greatest players, while Caruana is the first American to compete in the championship since the legendary Bobby Fischer in 1972.
Playing white, reigning world champion Carlsen narrowly avoided a defeat to his 26-year-old American challenger.
A draw fetches half a point and a win one point. The first to get 6.5 points in the 12-game contest wins.
Both players now have three points after six draws.
The seventh game in the 12-match tournament will begin on Sunday.
If the two are still level after 12 games, the title will be decided by a series of tiebreakers.