Spurs rallied after Ashkan Dejagah threatened to give René Meulensteen a winning start at Craven Cottage, courtesy of Vlad Chiricheș' equaliser and Holtby's thunderbolt.
A January summer signing from Schalke, the German has struggled to distinguish himself at White Hart Lane.
Holtby has also failed to capitalise when handed rare starts by André Villas-Boas, and had to settle for a substitutes' berth in west London on Wednesday night. But he emerged to ease the pressure on his coach.
Villas-Boas was full of praise for goalkeeper Hugo Lloris, however.
"It's fantastic for him: he needed a game like that for his confidence, Villas-Boas said. "This can be a big confidence boost for him going forward to make decisive saves and to have this impact on the game was important for him on a personal level.
"We brought him in last year with a view to strengthening, he fought for his place and for the team: he's a top, top goalkeeper who has great ambitions for himself and for the club.
"When called into action like that he showed all his qualities."