Tottenham Hotspur's title ambitions were dealt a severe blow as West Brom defender Craig Dawson's second-half equaliser put Leicester City within one victory of winning the Premier League.
Spurs looked set to narrow the gap to five points with three games left after Dawson bundled Christian Eriksen's teasing cross into his own net.
But he made amends by heading in Craig Gardner's corner from six yards.
Spurs are seven points adrift, meaning Leicester can win the title on Sunday.
Spurs, who hit the woodwork three times, must earn two wins and a draw from their remaining three games to stand any chance of winning their first Premier League title.
Leicester's unlikely ascension to Premier League title favourites has diverted some attention away from Tottenham's search for a first English title since 1961.
Mauricio Pochettino's men have played with freedom and fluency in recent months, thriving on their emergence as Leicester's only credible challengers.
But, while many have expected the Foxes to crack, it is second-placed Spurs who seem to have buckled on the final straight.
Spurs have only lost once in the Premier League since mid-January - a run spanning 14 matches - but dropping two points against a mid-table West Brom side, who have little to play for, could prove costly.
The home team oozed confidence in the first half, pinning Albion back inside their own half as they threatened to run riot.
However, the visitors rallied after the break - presumably after some stern words from manager Tony Pulis - to earn a point.
Spurs midfielder Dele Alli received a rapturous reception from the White Hart Lane crowd when he was handed his PFA Young Player of the Year award before kick-off.
But, by the final whistle, attention had been diverted to the ill-discipline which has occasionally crept into his game.
Alli may find himself facing a suspension after seeming to punch Albion midfielder Claudio Yacob in the stomach as they contested a first-half set-piece.
The England international has won plenty of admirers - not just those of a Spurs persuasion - in his breakthrough Premier League season, impressing with his mature displays.
Alli was instrumental in two of the moves that ended with Tottenham hitting the woodwork, calmly steering into Harry Kane's path in the opening stages and cleverly flicking to Eriksen, who then set up Erik Lamela's drive when Spurs led 1-0.
However, the 20-year-old has also shown a tendency to lose his cool - and his clash with Yacob, which was off-the-ball and later picked up by television cameras, could attract the attention of the Football Association's disciplinary panel.
Few people predicted anything other than a comfortable Spurs win against an Albion side struggling for form.
The Baggies had taken just one point from their previous five matches, including a limp 2-0 defeat at Spurs' bitter rivals Arsenal on Thursday.
Albion manager Tony Pulis insisted before kick-off that his side needed to "hold a flag up" for the competitiveness of the Premier League - and that is what they eventually did.
The visitors offered nothing in the first half, failing to muster a single shot on target as Spurs peppered efforts at their goal.
Albion keeper Boaz Myhill - making his first appearance since January for the injured Ben Foster - superbly tipped Eriksen's free-kick around his right-hand post, then diverted Kane's guided shot onto the same upright.
Another Eriksen set-piece also skimmed the crossbar as Spurs played with freedom in the opening 25 minutes.
But Pulis' side, now 13th, improved dramatically after the break, limiting Spurs to few clear-cut opportunities inside a tense White Hart Lane.
Tottenham boss Mauricio Pochettino on BBC Radio 5 live: "A second goal would have made it very different. In the first half we played very well and it will always happen in football when you allow the other team to believe you can concede.
"But I think I am still very proud for my players. We still need to believe. We know it will be difficult but we need to still believe and fight for these three games.
"We have a chance but it is true that now it is more difficult. We understand that this was a must-win and now we have to lift the players."
West Brom manager Tony Pulis:
"We just watched them dance around us at times but the great thing about the game was we came in just 1-0 down at half-time.
"But second half was much better - more like the performances we've been putting in away from home.
"I'm really pleased with the players. We lack a bit of quality at times but I can't fault their work-rate and they work within a shape that suits them."