Germany's Alexander Zverev reached his first Grand Slam final in the most difficult manner after fighting back to beat Spain's Pablo Carreno Busta in a tense US Open five-setter.

But Zverev showed incredible resolve to turn the match around and win 3-6 2-6 6-3 6-4 6-3 against the 20th seed.

Zverev will play Dominic Thiem or Daniil Medvedev in Sunday's final.

Austrian second seed Thiem, 27, faces Russian third seed Medvedev - who lost to Spain's Rafael Nadal in last year's final - in Friday's second semi-final at Flushing Meadows.

The final on Sunday will see a new Grand Slam champion crowned for the first time since the 2014 US Open when Croatia's Marin Cilic won.

The absence of Nadal and Switzerland's Roger Federer at the behind-closed-doors Grand Slam, plus the expulsion of Serb top seed Novak Djokovic for hitting a line judge with a ball, has given the chance for a new name to be etched onto a major trophy.

Switzerland's Stan Wawrinka was the last man outside of the 'Big Three' to win either the Australian Open, French Open, Wimbledon or US Open, following his win at Flushing Meadows in 2016.

The chances of Zverev becoming the newest Grand Slam champion looked slim after Carreno Busta's strong start.

But the former world number three is now one win away after a remarkable comeback in a semi-final which was neither high-quality or engrossing.

The German grinned incredulously as he waved his fists towards his box when Carreno Busta lamped a return into the net on the second match point.

"I knew I had to come up with better tennis and be more stable," said a relieved Zverev.

"I knew I had to play better, I've never come back from two sets to love but I'm happy to do it at this stage.

"I'm through to my first Grand Slam final and that's all that matters."

Zverev shows both sides of his ability in strange match

Zverev has long been touted as one of the next generation to eventually replace Nadal, Federer and Djokovic at the top of the men's game, but has too often showed a number of weaknesses which have led to many doubting his true credentials.

Here, the world number seven showed both sides of that make-up as he came back to win from a two-set deficit for the first time in his career.

Zverev lacked purpose and intensity in the opening two sets as a focused Carreno Busta executed the perfect game plan to take advantage.

Zverev's second serve is a glaring weakness in his game and it came to the fore again as things went from bad to worse in the second set.

Double faults in each of his opening two service games contributed to Carreno Busta opening up a 3-0 lead and he ended a one-sided set having won just three of 19 points on his second serve.

Another telling stat was Zverev's 22 unforced errors compared to just three from Carreno Busta's racquet. That led to former US Open semi-finalist Tim Henman describing Zverev's second-set performance as a "horror show" in his role as a television analyst.

Suddenly, Zverev loosened up in the third and began to look more like the player who won the ATP Finals two years ago. His service game improved drastically as he earned four holds to love and also cut down on the unforced errors.

Zverev maintained that level in the fourth as it became a question of whether Carreno Busta could handle his nerves.

The German continued to hit more winners than unforced errors in the decider, helping him break serve in the first and last games of the set.

In truth, it was a mixture of Zverev winning the match and Carreno Busta losing it.

More credit must go to Zverev, though, for showing an incredible resolve and refusing to panic when he faced defeat.