Madrid, Spain, 21 November 2019 | AAP

Australia is into the quarterfinals of the Davis Cup Finals in Madrid after victories by Nick Kyrgios and Alex de Minaur on Wednesday night built an unassailable 2-0 lead over Belgium.

De Minaur reinforced his reputation as one of the most exciting young players in world tennis, seeing off Belgium’s David Goffin to book Australia’s spot in the knockout stage of the revamped competition.

The 20-year-old Sydneysider beat the world No.11 6-0 7-6(6) to seal Australia’s first victory over Belgium since 1991.

The results sees Australia finish atop Group D,  setting up a clash with a talented Canada side on Friday (4am AEDT).

De Minaur won the first set in 26 minutes as Goffin struggled to find an answer to the red-hot Australian’s powerful return game.

The Belgian No.1 finally got on the board in the opening game of the second set, but de Minaur continued to have success from the back of the court.

The world No.18 broke again at 3-1 but Goffin, with his back firmly against the wall, the Belgian saved a match point when trailing 5-3 before breaking serve for the first time as De Minaur served for the match.

De Minaur then saved a set point and forced the tiebreaker, which he closed out to seal his 40th victory of the season.

“(That’s) Davis Cup emotions for you. Everything just means so much greater when you’re playing for your country,” De Minaur said.

”I’m happy that I was able to win for Australia. A win is a win however you may do it. I did it the hard way today and I’m very happy.”

It continues his brilliant 2019 in which he has won three ATP titles to rocket up the rankings into the top 20.

Earlier, Nick Kyrgios put Australia 1-0 up, beating the gutsy Steve Darcis 6-2 7-6(8).

Kyrgios largely kept his emotions in check to see off the wily 35-year-old — who has been Australia’s nemesis in two previous Davis Cup encounters — in a match delayed by more than two hours.

He powered through the opening set in 24 minutes and then held his nerve against a much-improved Darcis in a high-quality second set in front of a noisy crowd dominated by Belgian fans.

Once again Kyrgios held his serve throughout, hurling down 22 aces – including an incredible nine in the his final 10 service points to end the second set.

But Darcis refused to lie down and saved two match points – one from a 21-shot rally and the second from a 31-shot extravaganza.

Kyrgios did well to hold firm when it mattered and saved two set points to close out the win.

The final doubles rubber John Peers, partnering Jordan Thompson, retiring with what appeared to be an arm injury just one game into the match against Belgian pairing Sander Gille and Joran Vliegen.

Great Britain’s marathon 2-1 win against the Netherlands earlier on Court Three, which saw all three rubbers go the distance, kept Australia and Belgium waiting.