When the tennis world stirred from its brief December hibernation in 2016, nobody predicted the coups that happened at the top of the men's and women's games, let alone how seismic they've been. Novak Djokovic looked impenetrable. Serena Williams seemed indomitable. Both had a hold on the No. 1 ranking that stretched back years when they arrived at last year's Australian Open.

Djokovic was chasing his fifth major in the past 11 Grand Slams. Williams, who nearly had a calendar-year sweep in 2015, glided into the tournament hoping to win a 22nd Grand Slam singles title that would tie Steffi Graf's record and cementing Williams' own claim to Best Ever, if she hadn't already.

Yet, over the next 12 months, Scotland's Andy Murray and Germany's Angelique Kerber gave everyone a lesson on what it takes to thrive, not just survive, in the time of legends.

The striking thing is that none of it seems exotic. For both players, it starts with talent wed to an unstinting work ethic and fitness. It takes being a cold-eyed critic of yourself, too -- but offsetting that with a determined, often audacious self-belief that it's never too late to improve or to defy your critics. It requires understanding how chasing day-to-day excellence is what leads to big-picture success.

Both Andy Murray and Angelique Kerber have the confidence to hang on to their No. 1 rankings for a long time to come. Getty Images

It's all textbook stuff. But when it all comes together, it leads to remarkable results.

As Murray explained to the press at Wimbledon, "Before, when I won here [the first time in 2013], I was genuinely motivated solely, really, by the Slams. I think my results for the rest of the tournaments showed that, whereas now I feel a lot more motivated throughout the whole year and at all of the events. Before, sometimes maybe a couple of weeks before the US Open, my mind was already in New York. I wasn't thinking about that week, maybe in Cincinnati or something like that. I was distracted. Now I feel quite different about that throughout the year."

It showed.

With the big four in men's tennis crumbling into disarray around Murray last May -- when Roger Federer and Rafael Nadal's years were already truncated by injuries -- Murray lost to Djokovic in a rain-spattered French Open final. With the win, Djokovic became the first man in 48 years to hold all four Slam titles simultaneously. It was also his 11th win over Murray in their past 12 meetings. Yet Murray was undeterred.

"The reason Novak's beaten me a lot recently is that he's been the better player," Murray told The Guardian at the time. "But if I play him tomorrow, it's a bit like roulette. People think there have been five reds in a row; it's got to be black next time. But the roulette wheel doesn't know what's happened before."

Murray went on to win Wimbledon in July with Ivan Lendl just back for a second tour as his coach, while Djokovic lost an early-round shocker there.

Then before even leaving the All-England Club, Murray did something bold. He publicly announced he was setting out to seize the No. 1 ranking -- and then he did it, mounting a relentless late-season charge that reaped five titles and sent him rocketing by Djokovic, a career-long nemesis dating back to their parallel junior tennis careers.

It was quite a bookend to a year that started with Murray deadpanning, "I feel like I've been here before" after his fourth defeat in an Australian final to Djokovic.

By the end of the year, when Murray held off Djokovic to win the ATP Tour Finals in London and maintain his year-end hold on No. 1, Murray had won 24 straight matches. Despite a three-set setback to Djokovic this past Saturday in the Qatar Open final, Murray still holds a 780-point lead in the rankings.

Murray is the second oldest male player to arrive at No. 1 since 30-year-old John Newcombe in 1974.

And for Kerber, 28, she is the oldest to debut at No. 1 ever, three years older than Jennifer Capriati was when she first held the top spot in 2001.

But Kerber was a far bigger dark horse than Murray to make a rise to the top.

Until 2016, Kerber had never gotten to a Slam final in her previous 13 years on tour. Last year, she made three and won two of them.

Afterward, she was frank about the new altitude being an adjustment. Beating Williams in three scintillating sets in last year's Australian Open final or later hearing herself called the No. 1-ranked player in the world when she took the court for her US Open title match against Karolina Pliskova "still sounds a little crazy," Kerber admitted to the press in September.

No wonder. Kerber turned pro at age 15, but even by her sixth season in 2009, she had won a scant three WTA main-draw matches. She had 11 first-round tournament losses in 2011, too. But late that same year -- seemingly out of nowhere -- she began the 2011 US Open ranked 92nd and yet tore to the semis before losing to 26th-seeded Flavia Pennetta.

Kerber has always been considered talented. But amid the big peaks and a temporary return to the valleys even after winning the Australian, Kerber thinks she landed on what was similar between that brief taste of glory she had in 2011 and her sustained 2016 run.

She believes it was the domino effect of whipping herself into such great shape. She now knows "I can run all day," which freed her to play more aggressively and genuinely gave her the confidence to go for broke even in the biggest moments rather than backslide into being a counterpuncher who shrank from taking risks.

Angelique Kerber has embraced her position atop the WTA rankings. AP Photo/Wong Maye-E

It sounds simple, even unremarkable. But for Kerber, doing it was not. After toppling Williams at the Australian, Kerber regressed and lost five straight tournament openers, including her first match at the French Open. It was shades of 2011 all over again. But much like Murray, Kerber didn't sugarcoat the reason.

It was mostly in her head, she allowed.

"For sure, the pressure is a little bit bigger, not less, because I think that everybody is expecting [success] now that I won," Kerber said to the press at the WTA Finals in November. "After [the French Open] loss, I had so much pressure on my shoulders, and I told myself, 'OK, I will try to enjoy it now and not going out with the pressure.' My mentality changed a little bit."

Now look. The long road Murray and Kerber took to huge success doesn't stigmatize them any more. It dignifies them. Stamps them as extra special.

Both players (but especially Murray, who is competing in arguably the greatest era in men's tennis) ignored the downbeat conventional thinking that the players ahead of them were just too good to be overtaken. They ignored their own failures and believed success was just around the next corner.

Another great part about Murray's and Kerber's late-career booms is that they both say they've genuinely been able to savor the enormity and wonder of their breakthroughs.

"All the dreams came true this year," Kerber said in November.

And Murray?

"Sometimes I still can't believe it myself."