Novak Djokovic admits being blown away by his fans' fanatical backing after battling past Canada's Denis Shapovalov in an emotion-charged encounter to book Serbia's place in the ATP Cup semi-finals.

Playing his first match in Sydney in a decade, Djokovic sparked soccer-style scenes from the over-excited crowd before eking out a tense 4-6 6-1 7-6 (7-4) comeback win on Friday.

The world No.2's stirring win followed Dusan Lajovic's 6-4 6-2 rout of Felix Auger-Aliassime and propelled Serbia into a semi-final on Saturday against Russia.

In the Friday's other game between Belgium and Spain, David Goffin achieved what only one other tennis player has managed in the past six months – a victory over Rafael Nadal.

The fleet-footed Goffin scored the biggest upset of the ATP Cup with his 6-4 7-6 (7-3) win over the world No.1 on Friday night as Belgium levelled the quarter-final tie with Spain at 1-1.

Nadal had enjoyed an incredible run after Wimbledon last year, going 26-1 in matches with one walkover through injury.

The pair traded breaks in the first set before Goffin stole the advantage in the ninth game, ripping a backhand winner down the line and forcing Nadal into error off his forehand for a 5-4 lead. He then served out the set.

Things started no better in the second for Nadal, who hadn't lost a set in his first three matches of the ATP Cup.

Goffin broke in the opening game and looked set to close out the match when 4-2 and threatening on Nadal's serve.

Tennis: Nadal loses, while Djokovic's fanatical Serb fans cause consternation_1
Inset: David Goffin of Belgium (R) after his win over Rafael Nadal in Sydney. Photo: AAP 

The 19-time grand slam champion held his nerve, however, broke back and forced a tiebreak only for Goffin to dominate it 7-3.

At Ken Rosewall Arena it was also anything but routine for Serbia's Djokovic amid wild scenes against Canada.

Tensions boiled over midway through the second set when Shapovalov received a code violation for allegedly telling a Serbian heckler "f— you".

Chair umpire Carlos Bernardes called Canadian captain Adriano Fuorivia over to order him to calm Shapovalov down as he struggled to control the boisterous, mostly pro-Serbian crowd.

After dropping six straight games to concede the second set, Shapovalov did well to regain his composure and pile the pressure on Djokovic in the decider.

Feeling the heat, Djokovic remonstrated with Bernardes after receiving a warning himself for an audible obscenity in the sixth game of the set.

Despite the umpire demanding unruly fans behave or go home, Djokovic further fuelled emotions when he ignited the crowd after he held for 3-3.

He then sent Serbian supporters into a frenzy when he broke Shapovalov three games later, only to drop serve while serving for the match after a spectator fell ill and briefly delayed proceedings.

Djokovic offered the female fan a bottle of water, but it was the Canadian who needed cooling down after becoming enraged once again by spectators interrupting his serve during the third-set tiebreaker.

Vanquished but gracious, Shapovalov warmly embraced Djokovic at the net when finally succumbing after two hours and 40 gripping minutes.

"Brisbane had some amazing support (during the group stages) but this was a different level," Djokovic said.

"This has definitely exceeded our hopes and expectations, and we are super grateful to have this many Serbian people show up.

We knew that Serbian community is big in Sydney, but we didn't know that this many people would come. So loud … it was a Davis Cup-like atmosphere, really."

Djokovic said Shapovalov was "rightfully annoyed" by the crowd's at times unsporting antics.

But, having been on the end of pro-Roger Federer crowds for years, the seven-times Australian Open champion was happy to be the crowd favourite for a change and said the support helped him over the line.

"I mean, when you have most of the stadium backing you up after every point, of course it makes a significant difference in terms of how you feel on the court," Djokovic said.

"If the majority of the stadium is backing you up, it's easier. You find that strength. You find that motivation.

"When you're down, they lift you up. You feel more alert, more responsible because so many people are backing you up and supporting you that you have to deliver your best game.

"Of course in this kind of energy, you want to play in this kind of atmosphere all the time. But it's not always possible.

"That's why I try to cherish these kind of moments and take them with me anywhere I go."

-with AAP