A goal from Aleksandar Kolarov guided 10-man Serbia to a 1-0 win over Ireland on Tuesday, putting the Balkan nation on the verge of a place at next year's World Cup.
The win in Dublin leaves Serbia top of Group D, four points clear of Wales with two games remaining against Austria and Georgia.
Unless Serbia slips up, Wales and third-placed Ireland face a tough fight for a playoff place after the Welsh earned a 2-0 win over tenacious Moldova.
Both Ireland and Serbia started cautiously in a game which yielded few clear-cut chances. Ireland's Shane Duffy headed the ball into the net midway through the first half, but the strike was disallowed for offside.
Kolarov, an experienced full-back, ran onto a pass from Filip Kostic in the 55th minute and unleashed a fierce shot.
Serbia was reduced to 10 men after defender Nikola Maksimovic was sent off for a desperate lunge on Daryl Murphy in the 68th as the Ireland forward ran through on goal. Maksimovic got the ball but scythed down Murphy in the process, and the referee showed a straight red card.
Murphy appealed for a penalty in the 76th when he was held in the box, but no foul was given.
WOODBURN ON FIRE
Wales struggled to create many chances against a defensive Moldovan team until 17-year-old Liverpool player Ben Woodburn came on as a substitute.
After scoring the winning goal for Wales against Austria on Saturday, Woodburn revitalized his team against Moldova with smart passing and provided the cross for Robson-Kanu's diving header in the 80th.
Aaron Ramsey made it 2-0 for Wales, deep into stoppage time.
The match on Oct. 9 between Ireland and Wales in Cardiff looks increasingly likely to be a knockout game, with the winner claiming second place in Group D and a likely spot in the playoffs.
Before then, Wales must play fifth-placed Georgia, while Ireland faces Moldova the same day.
Fourth-placed Austria last played at the World Cup in 1998, and that 20-year wait looks like getting even longer.
Tuesday's disappointing 1-1 draw at home to Georgia was a missed opportunity to close in on the playoff places. It leaves the Austrians needing to win their last two games against Serbia and Moldova and hope that other results go their way.
Valeriane Gvilia gave Georgia the lead in the eighth minute in Vienna, but his goal was canceled out in the 44th by Louis Schaub.