Karen Khachanov and Andrey Rublev helped Russia open the inaugural men's tennis Davis Cup Finals with a 3-0 victory over defending champions Croatia in Madrid on Monday.

World number 17 Khachanov clinched the point for Russia by fighting back to beat Croatian number one Borna Coric 6-7 (4/7), 6-4, 6-4, after Rublev had earlier seen off Borna Gojo 6-3, 6-3.

"When you're playing not only for yourself, but for your country, you have to fight harder," said Khachanov.

Russia, who are without U.S. Open runner-up Daniil Medvedev due to exhaustion, could wrap up a place in the quarter-finals with victory over Rafael Nadal's Spain in their second Group B tie on Tuesday.

The 18 nations have been split into six groups of three at the Madrid-based Caja Magica stadium, with the pool winners and two best runners-up to play in the last eight of the first edition of the revamped tournament.

The teams faced off in two best-of-three-set singles matches and one doubles rubber in each tie.

Khachanov and Rublev completed the clean sweep later on Monday with a 7-6 (7/3), 6-4 doubles win over Ivan Dodig and Nikola Mektic.

Canada down Italy

Canada claimed a 2-1 victory over well-fancied Italy to strike first in Group F.

World number 12 Fabio Fognini of Italy slumped to a 7-6 (7/5), 7-5 defeat by Vasek Pospisil of Canada, ranked 150th, before Canada's rising star Denis Shapovalov edged out Matteo Berrettini 7-6 (7/5), 6-7 (3/7), 7-6 (7/5).

"I'm really happy to get the win against a player like Matteo who has had such a good season," said Shapovalov.

"I always love representing my country... It's not often you get to play as a team, so it's amazing."

Italy, who won their only Davis Cup title in 1976, were the favorites in Group F with Fognini and Berrettini both having enjoyed excellent seasons.

The 20-year-old Shapovalov saved three set points before snatching the first set in a tie-break, only for Berrettini to level the match in a second-set breaker.

A clash without a single break of serve, was decided by another tie-break, with Shapovalov reeling off the last three points to win after two hours and 52 minutes on court.

Berrettini and Fognini boosted Italy's hopes of making the quarters with a hard-fought doubles success over Shapovalov and Pospisil, winning 6-2, 3-6, 6-3.

Canada take on the U.S. on Tuesday, with Italy facing the Americans the following day.

Belgium triumph in singles, Colombia grab doubles win

Belgium made a victorious start in Group D with back-to-back singles wins against Colombia.

Ahead of his retirement, the 35-year-old Belgian Steve Darcis embraced his last Davis Cup campaign in Madrid. He became the first player to score a win at the Davis Cup Finals on Monday, defeating Colombia's Santiago Giraldo 6-3, 6-2 in just over an hour.

The South Americans are playing at the top level of Davis Cup tennis for the first time, having never played in the old World Group.

Belgian David Goffin, who was playing for his country for the eighth straight year, battled back from a set down to secure an overall win following a 3-6, 6-3, 6-3 victory over Daniel Elahi Galan in two hours.

But the world number one doubles pairing of Juan Sebastian Cabal and Robert Farah grabbed a historic win for Colombia by battling past Sander Gille and Joran Vliegen 6-7 (5/7), 6-4, 7-6 (7/3).