LONDON -- On the eve of the ATP World Tour Finals, Alexander Zverev was asked when we will see the Ivan Lendl effect on his game. "Not immediately, but hopefully the beginning of next year," Zverev said. Then he added, "We'll see this week as well."

Yes, we did.

As the 21-year-old Zverev celebrated his stunning victory against world No. 1 Novak Djokovic in the final at London's O2 on Sunday night, Lendl sat in the stands with his infamous stone-faced expression almost ready to crack.

He should have smiled. He deserved to smile. Zverev, who announced he had hired Lendl just before the US Open, followed up his win against Roger Federer in the semifinals with an even more unlikely -- but equally convincing -- win over the previously all-conquering Djokovic, 6-3, 6-4.

Lendl wasn't able to spend much time with Zverev heading into London because of wrist surgery and various commitments. But they've clearly leveraged their time together.

Earlier this week in the round-robin stage, Zverev was outplayed by Djokovic from first to last ball. But Zverev changed his game plan heading into Sunday's showdown, mixing attacking tennis with outstanding defense. His serve was smooth and on point.

When Lendl was working with Andy Murray, whom he helped to become a three-time Grand Slam champion, the little things made the most impact.

Remember when Murray broke down in tears on court after he lost to Federer in the 2012 Wimbledon final? Lendl told him that he would never feel as bad on the court again.

Less than a month later, Murray won Olympic gold and went on to win the US Open a few weeks after that. The next year, he won Wimbledon in front of his home crowd. In their second spell together, Murray added a second Wimbledon title and another Olympic gold medal.

With a reputation for brevity and an abrasive sense of humor, Lendl famously used to go home after matches rather than socialize with the Murray team. But he knows when to be a hard taskmaster and when to put an arm around the shoulder, even if it's metaphorical.

Under Ivan Lendl, Alexander Zverev won the biggest title of his career. Clive Brunskill/Getty Images

After Zverev's win Saturday, the young German was struggling to understand why sections of the crowd had booed him for calling a let during a point when a ball boy spilled the ball and moved forward to retrieve it.

"It was an emotional moment for me," Zverev told Sky Sports on Sunday night. "I was supposed to be the happiest guy on the planet, beating Roger in the semifinals here, and it was tough for me. I didn't feel like I'd done anything wrong. I felt like I'd done the right thing, but the crowd didn't appreciate it.

"As tennis players, we take it personally, this kind of stuff. [Lendl] just said, 'Look, it's nothing personal. They're all Roger fans; he deserves to have the most fans in the world.' He just told me, 'Go out here today and enjoy.'"

Lendl worked on getting Murray to stand closer to the baseline and be more aggressive; with Zverev, it was more about picking his moments to attack, something he had not done in his round-robin match against Djokovic earlier in the week.

"He obviously analyzed the match that I played with him a few days ago, told me a few things I had to do different," Zverev said shortly after winning the title. "I was more aggressive today. I tried to take the ball earlier. Those kind of things.

"Obviously, Ivan, the experience he has on and off the court, is amazing. That helped me, as well, to kind of play the two matches that I played back to back now."

After winning five titles in 2017, Zverev's victory Sunday was the most important trophy of his career and his fourth of 2018. He finishes the year ranked No. 4, and with very few points at the Grand Slams to defend in 2019, he will have strong hopes of going even higher and maybe winning one of the sport's biggest events.

"It means everything," he told Sky Sports. "This is the biggest title I've won in my career. I really hope this is just the beginning. I'm still young. I still want to improve on things -- this means a lot to me."

In his on-court speech, Zverev made a point of thanking his father, Alexander Sr., describing him as "maybe the best coach in the world."

Lendl might have something to say about that.