Cash struck his second goal of the campaign as The Reds showed admirable second-half fighting spirit, but it wasn’t enough and they tasted defeat for the first time in nearly four months.
Kevin MacDonald named an unchanged starting line-up for the third cup game on the bounce, with Oliver Burke returning to action for the Under 18s after his cameo for the first team against Blackpool on Saturday.
The visitors overcame Manchester United in the last round and their Premier League quality told in the early stages as they began positively, pressing The Reds at every opportunity and giving them little room to breathe.
But Forest remained undeterred and Burke nearly took advantage of slack goalkeeping but his shot from range was blocked, before Joshua Onomah had an effort from the edge of the area that trickled well wide.
Tottenham were having bright offensive spells and Shayon Harrison squeezed a shot away from just inside the are under heavy pressure from his marker, but Reds’ keeper Tim Erlandsson tipped wide as he searched for his fourth successive clean sheet.
Ismail Azzaoui had propelled his side to this stage of the competition thanks to his five goals so far, and he came very close to giving Tottenham the lead when the ball found its way to him at the back post, but Erlandsson smothered him before he had the chance to get the shot away.
Soon after, Onomah pace caused problems again as he surged forward but saw his left-footed strike go wide.
The vocal City Ground crowd then had something to shout about as Forest grew in confidence and Gerry McDonagh’s header hit the bar, before referee Paul Tierney brought the play back following a push by Burke. The Irish forward was involved again as he was brought down just outside the area, but Jack Kelly saw the resulting free-kick curl inches over Harry Voss’s bar.
As the first period progressed MacDonald’s men started to harry Tottenham in possession a bit more, sensing there were spaces to be exploited. Last Friday’s hat-trick hero Luke Thomas was brought on in place of the injured Tom Gamblen on the left of midfield but moments after Onomah was involved again, hitting a free-kick from 25 yards that Erlandsson was even to.
The Reds looked spritely in the opening exchanges of the second half, with Thomas having a run on the left, finding Walker who couldn’t turn around with enough venom and the ball was eventually cleared. Kelly had another free-kick that flew over following a superb attacking run from Matty Cash from midfield.
Erlandsson had to be at his best to deny an excellent fizzing long range strike from Harrison, who saw his shot palmed away to Azzaoui who couldn’t capitalise and Kelly eventually cleared.
Reds’ centre half Joe Worrall had to stand firm to clear a powerful crosss from Kyle Walker-Peters as Tottenham upped the ante to try and break the deadlock.
The visitors continue to pose dangers with their pace, and soon took a deserved lead. Right-back Kyle Walker-Peters sprung forward past Kelly and unleashed a powerful shot past Erlandsson into the bottom corner.
Encouraged and with their confidence on the up, Tottenham extended their lead a couple of minutes after. Azzaoui received the ball and struck a low shot that was beat Erlandsson’s dive to nestle into the corner.
But with Forest’s frequently shown spirit, they got back into the game thanks to a second goal of the season for Cash. The attacking midfielder calmly received the lay-off and hit a shot low into the bottom corner to get the crowd off their seats and believing once again.
The last quarter of an hour looked as if it would be open and Burke came close to latching onto a flick-on, but he couldn’t quite connect with it, with both defences under the cosh in periods.
MacDonald and Jack Lester opted to bring on the in-form Kasheme Walton to join the attack as Forest upped the tempo and began to go forward in their droves.
It was a valiant effort towards the end as The Reds desperately attempted to hold on to their superb unbeaten record but Tottenham edged through and snuffed out Forest’s threats.
Forest: Erlandsson, Yates, Kelly, Crookes (c), Worrall, Iacovitti, Burke, Cash, McDonagh (Walton 78'), Walker, Gamblen (Thomas 37' (Karo 78'))
Subs not used: Dearle, Burns
Goal: Cash 70'
Voss, Walker-Peters, Walkes, Amos, Maghoma, Carter-Vickers, Daly (Loft 70'), Stylianides, Harrison, Onomah (c), Azzaoui
Subs not used: Glover, Paul, Owens, Ross
Goals: Walker-Peters 63', Azzaoui 65'