Aleksandar Mitrovic netted a late equaliser as Wales were held to a third consecutive stalemate in World Cup qualifying as Serbia drew 1-1 in Cardiff.
Having previously seen Austria and Georgia fight back to claim a point apiece, Chris Coleman's side thought they had secured victory in Saturday's Group D encounter as they led with five minutes remaining.
For all Serbia's dominance, Wales had looked the more threatening throughout and scored through Gareth Bale after half an hour.
Bale flashed the game's first attempt over the top from around 25 yards, but chances were few and far between at both ends in a cagey opening period.
Filip Kostic sliced wide after Nemanja Matic had stolen possession from Joe Allen, while Williams stood tall to block Dusan Tadic's rasping drive.
Serbia winger Kostic then blazed over, but the hosts, who had struggled to find a foothold in the contest, soon led.
Hal Robson-Kanu took the ball from Matija Nastasic on the right and turned inside to find Bale, whose firm, low shot flew past the poorly positioned Vladimir Stojkovic into the net.
The two players involved in the opener then reversed their roles, but Robson-Kanu could not steer his header from Bale's cross on target.
Although Matic belatedly mustered a maiden effort on target for the visitors shortly before half-time, Wayne Hennessey was equal to his thump with a fine parry.
However, with Mitrovic starved of service, the away side's dominance counted for little, and, for all their hard work, no serious chances appeared forthcoming.
Wales instead almost secured the three points as Bale ran through and poked against the upright from Aaron Ramsey's pass, before Mitrovic struck.
Climbing to meet Antonio Rukavina's cross, his header clipped the post to deceive Hennessey and frustrate Wales.
