Roger Federer fell to a shock defeat to Dominic Thiem at the ATP Finals on Sunday and Rafael Nadal did likewise against Alexander Zverev last night.
Two of the top three seeds now face an uphill battle to make it out of the group stages at London’s O2 Arena.
And tennis pundit Daniela Hantuchova fears Swiss star Federer is not up to match sharpness.
“Dominic did what Sacha did last night and just took it away from Roger,” Hantuchova told Amazon Prime.
“He was playing very aggressive but at the same time stayed in the points when he needed to.
“He is not afraid to come in when he had a chance.
“I just felt that Roger was not as sharp as we are used to seeing him, especially indoors.
“It is his favourite court and hopefully the next match with the footwork he is more spot on.”
Nadal and Federer’s defeat will come as a boost to Djokovic, who is now in the driving seat to finish the year as world No 1.
“With [Nadal’s] loss Djokovic will be delighted because he knows it’s in his own hands now,” fellow pundit Greg Rusedski said.
“The problem Novak Djokovic has right now is he’s playing Dominic Thiem tonight and Thiem played brilliantly against Federer.”
Federer has never lost a second round robin match at the ATP Finals in all 16 appearances.
And he knows the importance of getting back to winning ways against Matteo Berrettini this afternoon.
He said: “My forecast, it’s a normal tournament from here on forward. Not allowed to lose anymore for me.
“That’s how it is every week of the year for the last 20 years, so from that standpoint there is nothing new there.
“Matteo, with his serve, with what he can do, obviously any opponent is dangerous here. He didn’t obviously have the best match today against Novak, you know.
“It’s probably also the toughest way to start off the World Tour Finals, playing Novak on the indoors, because he rarely has hiccups as well early on in tournaments. But, yeah, I’ve got to recover, I’ve got make sure I play better than today, and hopefully I can win that match.”