Zverev was on the brink of defeat against No.20 seed Carreno Busta, from Spain, in New York, but the German star produced a stunning rally to emerge victorious 3-6 2-6 6-3 6-4 6-3 on Saturday (AEST).

After earning his first win from two-sets-to-love down, Zverev, the youngest Major finalist since Novak Djokovic in 2010, is now looking to become the first German Major champion since Boris Becker in 1996 as he awaits Dominic Thiem in the decider.

No.5 seed Zverev, who reached the Australian Open semi-finals earlier this year, is also the first German man through to the US Open final at Flushing Meadows since 1994 after three hours, 22 minutes on court.

Carreno Busta, who benefited from world No.1 Djokovic's default in the fourth round, made a flying start and led 5-1 in the opening set before dropping his own serve for the first time.

Zverev was too far back in that set to make it close though, and he was desperately flat in the second set too, sliding two breaks behind without making Carreno Busta fight especially hard for that enviable position.

Carreno Busta, a two-time US Open semi-finalist, wobbled slightly, but he could afford to, and he soon held a two-set cushion.

But Zverev refused to surrender and he showed more verve and attacking intent to finally break his opponent's serve at a meaningful point in a set and lead 3-1 in the third, but that was immediately clawed back.

The inclination was to think that might have marked the end of the Zverev fightback, given he had shown such little dynamism through the first two sets, but he broke again and soon led 5-2 as he stayed alive in the match.

Zverev's serve and ball-striking was more assured than at any point in the match, with Carreno Busta for the first time given serious pause for thought, the momentum beginning to turn.

That was only accentuated when Carreno Busta dropped serve in game three of the fourth set, but a wretched service game from Zverev brought the fourth set back on level terms.

However, Zverev earned a pair of break-point chances in a tense seventh game and while Carreno Busta saved one, he was unable to save the other as the former's deep approach shot set up an emphatic overhead smash for the break and 4-3 lead.

Carreno Busta then saved three set points in a battling display, but there was no denying the rallying Zverev, who levelled the match at two-sets-apiece with an ace to force a decider.

After a medical timeout for back treatment, Carreno Busta emerged trying to halt Zverev's momentum, however, the latter was too hot to handle with a match-high 18 winners in the set to complete a memorable fightback as he became the first German grand slam finalist since 2003.

MATCH STATS

Zverev [5] bt Carreno Busta [20] 3-6 2-6 6-3 6-4 6-3

WINNERS/UNFORCED ERRORS

Zverev – 71/57
Carreno Busta – 37/43

ACES/DOUBLE FAULTS

Zverev – 24/8
Carreno Busta – 4/1

BREAK POINTS WON

Zverev – 7/21
Carreno Busta – 7/16

FIRST SERVE PERCENTAGE

Zverev – 62
Carreno Busta – 71 

PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE

Zverev – 78/37
Carreno Busta – 65/42

TOTAL POINTS

Zverev – 148
Carreno Busta – 138