That win over Nadal in Wimbledon 2018 was the lifeline he needed. Felt like a miracle in a way. Everything looked bleak before that as you said. Arguably the biggest win of his career (against a great Nadal) or among his top 5 wins at the very least. Could have easily gone Nadal's way and got his 3rd Wimbly.

 

Beginning of 2018, Novak looked completely washed.

He had missed the back half of 2017 due to injury.

2018 starts and he has 3 consecutive losses to these mugs:

He literally had surgery in February 2018 or thereabouts. Some of those early losses were just coming out of rehab and getting used to it.

This is the news from back then:

BELGRADE (Reuters) - Novak Djokovic has undergone an elbow operation in an attempt to cure an injury that has troubled him for the past two years.
The 30-year-old Serb said he had agreed to surgery after last month's Australian Open when he was treated on court in losing to unseeded South Korean Chung Hyeon in the last 16.
"I agreed with my team that I would try different methods after I finish in Australia and a few days ago I accepted to do a small medical intervention on my elbow. It seems like I am on the good road now to full recovery," he posted on Instagram.

The 12-time grand slam champion had hoped the extended break he took last year might solve the problem but said he decided on more drastic action after still experiencing pain in Melbourne.
"I've been carrying this injury for the past two years, and during this time I've been seeing many doctors," said Djokovic, who before last month had not competed since losing at Wimbledon last year.
"I took six months off last season hoping to come back fully recovered, but unfortunately I still felt pain," he said.
"It's quite a journey this one, I have to say. I'm learning a lot and for that I'm grateful. I've always taken care of my body and looked for the most natural ways to heal, and my body has rewarded me with some incredible years on tour.

 

He simply returned to form during the middle of the season. There have been other instances though not equally impressive.

Take Andre Agassi's 1999 season.

16-05-1999	Dusseldorf	Clay	RR	LR	Nicolas Escude d. Andre Agassi	4-3 RET	Stats
10-05-1999	Rome Masters	Clay	R16	L	Patrick Rafter (4) d. Andre Agassi (14)	6-1 7-6(4)	Stats
15-03-1999	Miami Masters	Hard	R64	L	Dominik Hrbaty d. Andre Agassi (9)	1-6 6-3 6-2	Stats
01-03-1999	Scottsdale	Hard	SF	LR	Jan Michael Gambill d. Andre Agassi (3)	5-4 RET	Stats
08-02-1999	San Jose	Hard (i)	R16	LD	Cecil Mamiit (Q) d. Andre Agassi (2)	0-6 6-6 DEF	Stats
18-01-1999	Australian Open	Hard	R16	L	Vincent Spadea d. Andre Agassi (5)	6-1 7-5 6-7(3) 6-3

Andre endured six losses in 18 matches before he won his only Roland Garros title. He went on to reach the final of Wimbledon losing to Sampras and followed it with US Open win. He also reached the final of ATP world championships (beating Sampras along the way) before losing to Pete again and ended the year as no.1.

 

He didn't play any better than before, he simply feasted on the carcasses of the lost generation.

 

I agree with you in the post covid era, but i think calling 2018, a year where we still had a very good Nadal and Fed who just won AO back to back is far from being a vulture year for Novak. It might not have been his most incredible year performance wise (2011 and 2015) but the way he fought back from a potentially career ending injury to win 2 slams in the back half of the year was incredible. The field hadn't thinned out yet, even though it wasn't as strong as it used to be, so he deserves respect for having the year he did.

 

Beginning of 2018, Novak looked completely washed.

What turned around his career at that point?

Roger.

Roger has his renaissance in 2017/18. This period likely instilled a sense of urgency, if Novak didn't act fast, Roger (now at 20 Slams) might put the all-time Slam record out of reach.

After months of rebuilding his form, Novak went on to win 4 slams across 2018/19, his most dominant stretch since 2015/16.

A hyper-competitive athlete like Novak thrives on rivalry and legacy. Seeing Roger reemerge likely reignited Novak’s motivation

 

After his meltdown in the post-match press conference after losing to Cecchinato at RG where he said "I don't even know if I'll play the grass season" or something similar along those lines.

He played Queens some 2-3 weeks after the RG loss and lost a tight 3 setter in the final to Cilic (had a MP but it was on Cilic's serve), he started looking in better form compared to the first half of the year.

Fast forward to 2018 Wimbledon - I believe there are 2 key matches that kickstarted Djokovic back to form.

• The first one was against the Brit Kyle Edmund in the 3rd round where Edmund lost a point 3 ways (ball bounced twice, Edmund runs into the net, Edmund's shot was out) yet the umpire totally screwed Djokovic over and gave the point to Edmund, the crowd also got Djokovic riled up which is a big no-no, I can still visualise Djokovic's roar with his eyes and mouth wide open and giving the crowd the angry glare
  
  
• The second match was the SF vs Nadal, that was the BIG occasion and I would say the turning point in the 'slam race'. Nadal was up 17-12 on Djokovic at the time, Nadal undoubtedly gets to 18 if he beats Djokovic there since Anderson was a useless mental midget when facing Nadal (plus he was gassed after that marathon borefest of a SF vs Isner). Instead of a +6 advantage for Nadal after 2018 Wimbledon (18-12), that advantage shrinks to +4 (17-13).
  
  
• Another match that gave Djokovic confidence was finally defeating Federer in the Cincy final that year (after previously losing to Federer in the 2009, 2012 and 2015 Cincy finals, also lost to Murray in the 2008 and 2011 Cincy finals), Djokovic won Cincy for the 1st time ever (he was definitely relieved after being runner-up on 5 previous occasions) and completed the Career Golden Masters that day.

Djokovic had a pretty comfortable US Open draw, he was drawn to meet Federer in the QF and I picked Djokovic to win that match had it occurred. It's not Djokovic's fault that Chokerer showed up against Millman in the 4th round.