The man with the best motor in tennis added some power to his package, but even that was not quite enough to get Alex de Minaur and Australia over the finish line against Spain, led by world number one Rafael Nadal.

Key points: De Minaur took the first set off Nadal before falling in three sets

De Minaur took the first set off Nadal before falling in three sets Earlier, Nick Kyrgios lost for the first time in the tournament, going down in straight sets to Roberto Bautista Agut

Earlier, Nick Kyrgios lost for the first time in the tournament, going down in straight sets to Roberto Bautista Agut Spain will play Serbia in the final of the team tournament

De Minaur lost a thrilling 4-6, 7-5, 6-1 match to Nadal, as the Australians fell at the semi-final stage of the ATP Cup, with Nick Kyrgios earlier delivering his poorest match of the tournament, going down to Roberto Bautista Agut.

Kyrgios's 6-1, 6-4 defeat had left de Minaur needing an unlikely victory to push the match to a deciding doubles clash.

But, adopting a similarly aggressive game plan to Belgium's David Goffin, who downed Nadal on Friday night, de Minaur was outstanding, hitting winner after winner to put the 19-time major champion on the back foot.

Nick Kyrgios enjoyed a superb ATP Cup leading into the semi-final against Spain. ( AAP: Craig Golding )

"Obviously I knew the task at hand, and I knew that if I wanted to hurt Rafa I was going to have to play like that," de Minaur said.

Breaking Nadal on his first service game, he held firm to take the opening set and went toe-to-toe with the Spaniard for much of the second.

After a break-point chance at 5-5 to serve for the match, things then started to unravel for de Minaur.

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Nadal snatched the set with his only opportunity as part of a run of six consecutive games to effectively seal the match.

The irrepressible world number 18 pinched a break back but it was not enough as Nadal avoided losing back-to-back matches for the first time since October 2016.

Nadal said while he was down on form it came as no surprise that de Minaur pushed so hard.

"He's young. He has a lot of energy. He plays with a lot of passion. Sometimes even too much," he said.

Kyrgios picks bad time to drop first match

Speaking of too much passion, Kyrgios was his flammable self as his perfect record at the ATP Cup was undone by Bautista Agut.

After impressive wins over Jan-Lennard Struff, Stefanos Tsitsipas and Cameron Norrie, as well as a decisive doubles win alongside de Minaur in the quarter-final against Great Britain, Kyrgios struggled from the outset and never recovered.

Down 5-0 in the opener, the best the world number 29 could do was hold serve to ensure he could start up the second with ball in hand.

A tighter contest ensued early in the second before Kyrgios exploded at a controversial let call at 2-2.

Facing a break point at 30-40, Kyrgios thought he had saved it with an ace. The crowd thought so too, cheering so raucously that the umpire's call of "let" was missed.

Kyrgios was frustrated as he struggled against Roberto Bautista Agut. ( AAP: Craig Golding )

A fault and a forehand error in the ensuing rally gave Bautista Agut the break and he was in control from that point on, as Kyrgios destroyed his racket on the Ken Rosewall Arena playing surface.

Chris Guccione and John Peers also lost the dead-rubber doubles against Feliciano Lopez and Pablo Carreno Busta in a super tiebreak as Spain completed the clean sweep.

Spain will play the Novak Djokovic-led Serbia in Sunday night's final after the Serbs beat Russia 3-0 in the other semi-final.

AAP/ABC