Filip Krovinovic's curling effort bent just beyond the fingertips of Bartosz Bialkowski for West Brom's opener

West Bromwich Albion extended their lead at the top of the Championship to four points with a hard-fought victory over Millwall in stormy conditions at The Den.

In an ebb-and-flow first period, visiting midfielder Filip Krovinovic curled in from the edge of the box to give West Brom the lead, and Kyle Bartley passed up a golden chance to add a second when he flashed wide.

Livewire winger Jed Wallace was the main Millwall outlet, and forced a smart save from Sam Johnstone with a curling free-kick before crashing a shot over the top when well placed.

The second half saw the Baggies dominate possession and play, but they had to wait until late on to sew up the points when Dara O'Shea headed in Kamil Grosicki's corner to put the result beyond doubt.

Wins for Fulham and Nottingham Forest on Saturday had compressed the top end of the second tier and this win gives Slaven Bilic's Albion a little bit of breathing space at the summit.

It was made even more impressive against a Millwall side whose home form has been their bedrock, losing just twice before Sunday's weather-affected encounter.

The swirling wind, patches of rain and a clotting surface failed to hamper the excitement as both sides contributed greatly to a lively contest.

Those conditions severely tested technique, with any loose touch threatening to cause havoc, as it did when West Brom forward Matheus Pereira latched onto one such lapse and rolled his shot onto the Lions' woodwork in the first five minutes.

Bilic's side adapted best to the conditions, even succeeding at times to pull off their one-touch style, and it took the reflexes of Millwall goalkeeper Bialkowski to keep the scoreline down - denying Hal Robson-Kanu one-on-one and turning away another Pereira shot.

There was also evidence of the depth available to them, as Matty Phillips and recent signing Grosicki made cameos from the bench, the latter swinging in a near-post corner which O'Shea guided in for his first Albion goal.

Even basic set-pieces were made difficult as the wind whipped around The Den

Gary Rowett's Millwall could claim a few first-half rounds - to borrow boxing parlance - having caused problems for their visitors in the opening 45 minutes.

Wallace was a menace going forward and Republic of Ireland under-21 player Jayson Molumby was another figure of influence with the ball.

However, after the break they spent large periods under pressure, but refused to buckle further when West Brom threatened to cut loose and remained a danger from set-pieces.

The league leaders have tough games against Reading, Forest, Bristol City and Preston before the end of February, while Millwall - who are five points off the play-offs places and have their own lingering promotion hopes - have Fulham, Preston and Wigan to come.

Millwall boss Gary Rowett told BBC Radio London:

"You have to first give credit to our opponents, they were very, very good and showed alongside Leeds that they're a top side and make it difficult.

"We're the sort of side that has to start well and at tempo and we've got to be at our best to make games like this difficult for any opponent. We didn't do that, we started a bit naively, we made the game really nice for some reason early on which was quite surprising and that set the tempo for what was to come.

"They opened us up too easily for my liking, we weren't solid enough and made it difficult from the start. We didn't really get out of that, we had a few moments where Jed [Wallace] had a few half-chances, but didn't play with enough composure or quality to make it a 10-15 minute spell. It was a disappointing performance today for different reasons."

West Bromwich Albion boss Slaven Bilic told BBC WM:

"The performance is what pleases me the most, it is why I'm emotional now. I said to the guys, it has to be like this, this is the minimum standard now we have to do in a physical way. This makes you happy and confident.

"To come here and not just match them, but better them in a physicality against a team who are best in the league for that, that means something.

"If it's like that you have more chance to get a result. I'm extremely pleased with everybody, every single part of the squad. We were good.

"It was a well-taken first goal and Filip [Krovinovic] was very good in defence. It wasn't only the goal, how much he kept the ball and created chances, it's also the defending. He was winning balls in the air and that's determination and character."