Andy Murray says he "didn't deserve" to beat Dutch world number 179 Tallon Griekspoor after surviving a scare to give Great Britain a winning start at the inaugural Davis Cup finals.

Murray, 32, won 6-7 (7-9) 6-4 7-6 (7-5) in their group opener in Madrid.

"I'm obviously relieved just now," the Scot said. "I fought extremely hard but he dictated a lot of the points."

The group-stage tie continues when British number one Dan Evans plays Robin Haase later on Wednesday.

Jamie Murray and Neal Skupski are then set to play in the doubles to round off the opener.

Britain, who won the 2015 Davis Cup in the previous format, have been drawn against the Netherlands and Kazakhstan - who they face on Thursday - in the group stage.

Eighteen nations are contesting a football-style knockout tournament for the first time to determine the Davis Cup champions, with the winners of the six groups and the two best-placed runners-up reaching the quarter-finals.

Murray finds Griekspoor's hitting hard to handle

Former world number one Murray has not played a competitive match since winning the European Open title in Antwerp more than a month ago and he appeared to struggle physically at times against Griekspoor.

Murray revealed before the tournament he was at the heaviest weight of his career as he took a couple of weeks off to spend with his newborn son, joking with the media in Spain on Tuesday that he still needed to shift "the last couple of kilos".

While that may not have been the contributing factor for a sluggish first-set performance, there was a lack of match sharpness as he found the big-hitting Griekspoor hard to handle.

The 23-year-old Dutchman may have only won two ATP Tour matches in his career but they came against three-time Grand Slam champion Stan Wawrinka and Russian world number 17 Karen Khachanov, illustrating his capability of causing problems for the very best.

Eleven aces and some impressive first-serve statistics in the mid-80s rocked Murray in the first set, before the Scot showed signs of improvement in the second - although far from his best - to level the match.

Twice Murray looked beaten in the final set, trailing 4-1 and again by the same scoreline in the tie-break, only to dig deep into his reserves to fight back.

A blocked forehand from deep behind the baseline, with Murray almost standing next to the line judge, was almost beyond belief and helped him level at 4-4 to ultimately swing the match back in his favour.

Although Griekspoor won the next point, the momentum remained with Murray who sealed a memorable win when the Dutchman sliced into the net.

Murray pointed to his heart as he celebrated, the gesture signifying the resilience which has helped him win the sport's biggest prizes and resurrect a career which even he thought was over after "life-changing" hip surgery in January.

"I fought, put every ball in and had to find a way to win. That's what I did," Murray said.

Brilliant atmosphere & thrilling finale

Barcelona footballer Gerard Pique has overseen the transformation of the 119-year-old competition into the season-ending finals, which has been met with fierce criticism from some and doubts about the atmosphere such a format would create.

While Spain's opening match on Tuesday night was unsurprisingly watched by a partisan and full crowd at the Caja Magica, some matches have struggled to attend spectators.

Britain's opening tie against the Dutch was not expected to be one and so it proved as about 1,000 Union Jack-clad fans created a boisterous atmosphere on a 2,500-capacity third show court which was about three-quarters full by most estimates.

The crowd was given plenty of entertainment by Murray and Griekspoor in a gripping match full of ebbs and flows, a final-set tie-break providing a thrilling finale which eventually separated the pair.

Supporters from both nations were left standing on their feet and roaring their player towards victory in the tie-break, with Murray sealing the type of win he has earned throughout his illustrious career after two hours and 51 minutes.

"The atmosphere was one of the things players were worried about in a neutral venue but both sets of fans were fantastic," Murray said.

The new Davis Cup format

Eighteen nations are split into six groups of three, with the group winners and two best-placed runners-up progressing to the quarter-finals.

The two semi-finals will be played on Saturday, 23 November with the two winners going through to the final on Sunday, 24 November.

Schedule - who plays when?

Group stage: Monday, 18 November - Thursday, 21 November

Quarter-finals: Thursday, 21 November - Friday, 22 November

Semi-finals: Saturday, 23 November

Final: Sunday, 24 November