Full-time from The Den
The Reds took the lead against the run of play with Jamie Paterson's 34th-minute strike only for sub Williams to haul Millwall level with a sensational 50-yard lob with 14 minutes remaining. But any hopes Neil Harris' charges had of nicking victory were shattered when Lars Veldwijk thumped home an 88th-minute winner to condemn The Lions to their first home defeat since February.
Millwall manager Harris had stressed that the League is his No.1 priority this season prior to kick-off and as a result, tinkered with his team making seven changes to the side that started against Sheffield United at the weekend and handing full debuts to Tom King and Jamie Philpot.
Forest manager Phillipe Montanier also made changes to his starting XI - nine in all - as both managers took full advantage of the opportunity to rotate their respective squads.
Lions striker Philpot was soon involved in the thick of the action, latching onto a headed clearance and hitting a first-time volley from 25 yards that was deflected for a corner. Ferguson's resulting delivery from the right was headed back across goal by Byron Webster to fellow centre-half Nelson, but the latter's cushioned effort was comfortably collected by Forest keeper Stephen Henderson.
Nice combination play between Ferguson and Wylde stretched the Forest backline who were grateful when the latter's final delivery evaded a crowd of players in the penalty area.
On 19 minutes Wylde's drilled square pass was miscontrolled by Abdou but quickly picked up by Onyedinma, who in turn produced some deft footwork and then let fly with a fierce drive that Henderson did well to gather low to his left.
Forest were well contained by The Lions, who looked the more likely of the two teams to break the deadlock as the game headed towards the midway stage of the first half.
Wylde dragged a long-range effort wide of the mark, Philpot was unable to get a clean connection on Mahlon Romeo's inviting delivery and then a low shot from Aiden O'Brien was gathered by the keeper as Millwall continued to press for the opening goal.
However it was the visitors who scored against the run of play 11 minutes before the break when Paterson was on hand to slot home from close range after King had parried Veldwijk's header from Jamie Ward's cross.
Millwall twice went close to hauling themselves level within five minutes of conceding, but Wylde lashed the ball high over the bar after Ferguson's free-kick was half-cleared and then Nelson's powerful downward header from another Ferguson set-piece, this time a corner, bounced the wrong side of the post and behind for a goal-kick.
Harris was forced to make a substitution in the 44th-minute when Onyedinma was forced off six minutes after he had been on the receiving end of a crude challenge from Damien Perquis.
On came David Worrall and The Lions sub so nearly created the equalising goal seconds later when his dinked ball over the top fell invitingly for Abdou right in front of goal, but the midfielder's shot was blocked by Henderson. The ball fell invitingly for Wylde, but his follow-up was also blocked and the danger averted as Forest held on to their slender lead going in at the break.
Montanier introduced the 6ft 5in frame of Oliver Burke at the start of the second-half for James Thorne, but it was the home side who again enjoyed the balance of possession in the opening stages after the restart.
Wylde did really well down the left before finding Worrall with a cross, but the latter skied his effort over the bar and then Ferguson's teasing delivery was headed towards goal by Philpot, but lacking any real power to test the keeper.
Den boss Harris made his second change of the night just before the hour mark with Steve Morison replacing O'Brien and it didn't take long for the experienced front man to make his presence felt with an important header to clear Henri Lansbury's inswinging corner.
Forest's Veldwijk had two great opportunities to double his side's tally only to be thwarted by a couple of superb pieces of defending. There looked to be no way that Veldwijk could fail to score when he rounded keeper King and had the goal at his mercy, but there was Webster to make a perfectly timed tackle and nick the ball as the Forest man prepared to shoot.
Just as important, and timely, was the interception by Romeo as he toed away the ball as Veldwijk broke through.
Harris responded with his third and final change of the night, and what an inspired substitution it proved to be. Ben Thompson was the man to make way and on came Williams who took just three minutes to fire home his spectacular equaliser, picking up the ball in his own half, nudging forward and then, having spotted Henderson off his line, dispatched a magnificent lob worthy of gracing any stadia.
That should have been the cue for The Lions to push on, but it was Forest who cranked up a gear. Lansbury fired a free-kick over at the other end and it took a marvellous block from debut making keeper King to keep out Ward's angled drive from close range.
The visitors' pressure paid off with three minutes remaining when the dangerous Veldwijk was played in by Burke and drilled an unstoppable shot low into the bottom corner.
It was a cruel blow for The Lions, but Harris' new-look side could hold their heads high after giving their all in an engaging encounter.
Millwall: King, Thompson (Williams, 73mins), Onyedinma (Worrall, 44mins), Wylde, Romeo, Nelson, Webster, Ferguson, O'Brien (Morison, 59mins), Abdou, Philpot. Subs not used: Archer, Martin, Craig, Twardek.
Booking: Ferguson (79mins)
Scorer: Williams (76mins)
Nottingham Forest: Henderson, Lichaj, Mills, Perquis, Lam (Pereira, 67mims) Paterson (Cash, 89mins), Lansbury, Vaughan, Ward, Thorne (Burke, 46mins), Veldwijk. Subs not used: Smith, Mancienne, Grant, Vellios.
Scorers: Paterson (34mins), Veldwijk (88mins)
Att: 4,009 (away 707)
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