The race for the three promotion places in Division Two is hotting up after the leading pair both lost on Saturday.
South Shields, who went unbeaten in all competitions for 21 games, have now lost their last two league matches, both against top-four sides.
Ryhope CW were the first to come out on top last weekend and, on Saturday, Billingham Synthonia enhanced their chances of making an immediate return to the top flight when they won with an only goal at Central Avenue.
The Mariners suffered a double whammy in the closing stages. They had Julio Arca sent off eight minutes from time for fouling Macaulay Langstaff when he was through on goal.
And, with just a minute remaining, they conceded the winner to Sam Garvie, who cut inside a defender and fired past goalkeeper Gareth Young.
Shields still have a six-point lead because second-placed Ryhope CW also went down to defeat, their second in three matches.
Gary Pearson’s men succumbed to visitors Chester-le-Street following a five-goal thriller.
Damien Stevens drove the Wearsiders into a fifth-minute lead, but the Cestrians then built up a two-goal cushion courtesy of Lee Mole, Michael Hepplewhite and Lee Heslop.
With 10 minutes left, Conor Winter threw Ryhope a lifeline, but Chester held on and are now only a point behind second-top Ryhope, with Billingham Synthonia fourth, on equal points.
Team Northumbria, in fifth position, lost ground when they conceded a three-goal lead at Hebburn Town.
Sammy Perez and Gerard Richardson, with a brace, looked to have secured all three points for the visitors, but Hebburn roared back to level at 3-3 through Alan Pegram, Chris Feasey and Calvin McMahon, who equalised with a 91st-minute penalty.
Impressive Easington Colliery are also in the promotion mix and moved up to sixth place with an easy 5-1 win at Alnwick Town, where Aiden Goodey (2), Joe Kerridge, Jack Devlin and Michael Watson were the scorers.
Birtley Town’s problems at the bottom of the table continued when Crook Town eased their relegation fears with a crucial 2-0 victory, thanks to strikes by Damon Ellis and Callum Lough.