By Martyn Herman
LONDON (Reuters) - Twenty four hours after being booed for having the audacity to take down a tennis great, Alexander Zverev returned to topple another one as he stunned Novak Djokovic to win the ATP Finals title on Sunday.
Zverev, billed as the player most likely to lead the sport into a new golden era once the holy trinity of Roger Federer, Djokovic and Rafael Nadal have hung up their rackets, took full advantage of an off-key Serb to win 6-4 6-3.
Hamburg-born Zverev had been shamefully booed and heckled on Saturday by a Federer-worshipping crowd having beaten the six-time champion after a controversial second-set tiebreak.
Against world number one Djokovic, however, the 18,000 fans wedged into the cavernous O2 arena were roaring their support as he produced a level of tennis his opponent could not match.
He struck 20 winners to Djokovic's seven -- the last of which was a backhand to end a one hour and 19 minute duel.
"I really can't describe it. It is the biggest title I have ever won," Zverev said on court after becoming the first German to win the ATP's most prestigious title -- often referred to as 'the fifth slam' -- since Boris Becker in Frankfurt in 1995.
In becoming the youngest ATP Finals champion since Djokovic, also 21, in 2008, Zverev sent out a clear message that he is ready to spend a considerable amount of time at tennis' top table and start accumulating the game's biggest prizes.
Djokovic, whose error-strewn display was a shock after a dominant week in south east London which included an easy round-robin win over Zverev, sportingly walked around the net to congratulate his conqueror after match point -- the German still laying on his back in a state of disbelief.
Later, Djokovic told reporters: "There's a lot of similarities in terms of trajectory of professional tennis, in our careers. Hopefully he can surpass me.
"There's a lot of time ahead of him. (I) Wish him to stay healthy and obviously win a lot of titles."
Zverev, younger brother of Tour pro Mischa and now coached by eight-times major champion Ivan Lendl, is yet to go past the quarter-finals of a Grand Slam.
That is still the test next year but on Sunday he displayed an added sense of maturity, to go with the natural flair and easy power that have marked him out as a future number one.
His forehand, often regarded as a weakness, was armour-plated and he showed a refreshing willingness to attack the net -- a skill he perhaps has learned from his brother.
Most impressive was that he often prevailed in the long, attritional baseline rallies in which Djokovic has used as the foundation for 14 Grand Slam titles.
The opening eight games resembled a boxing match with both players probing for weaknesses -- although it was evident that Zverev's packed the more powerful punches.
Djokovic, winner of 35 of his last 37 matches during a dominant second half of the year which propelled him back to world number one, had not dropped serve all week and had faced only two break points in his four previous matches.
But at 4-4 he wavered as he yanked an edgy forehand halfway up the net to give Zverev the chance to serve for the set.
Zverev took his opportunity in emphatic fashion -- blasting down three consecutive aces then winning the set when another Djokovic forehand flew long.
An increasingly-ragged Djokovic then dropped his opening two service games of the second set, either side of breaking Zverev for the first time as the German played a rare loose game.
The crowd still expected a Djokovic fight back but it never materialised and at 3-5 the 31-year-old crumbled, serving a double-fault to help Zverev earn two match points.
He saved the first but was left standing as Zverev conjured a stunning backhand to cap a brilliant display.
(Reporting by Martyn Herman; Editing by Christian Radnedge)