Having met for the first time in a friendly in the USA in 1963, the clubs have since played 10 times in competitive games. The first meeting took place in 1966 at the quarterfinal stage of the European Cup, with the champions of Portugal having reached the competition's final four times in the previous five seasons, winning it in 1961 and 1962.

Unbeaten at the time in European competition on their own ground, the Stadium of Light, no team had been as consistent as Benfica in world football during the 1960s. Their side provided the majority of the Portuguese national team and remained practically unchanged for years.

United hammered Benfica over two legs, winning 8-3 on aggregate.

"We knew that we had to make the most of home advantage in the first leg yet Eusebio, their greatest player, crossed for Augusto to head in and give them an early lead," says Paddy Crerand, who played in both legs. "We showed our spirit by coming back with goals from Herd and Law to lead 2-1 at half-time. The football was so good that the fans separately applauded both sets of teams back onto the pitch for the second half."

With half an hour to go, Bill Foulkes made it 3-1 with a header but Benfica pulled another goal back through Torres. The headline in The Times the following day summed it up: "Precarious lead plucked from a magnificent match."

"One goal in the bank may not be good enough for Manchester United," the report continued. "But at least this night cannot be taken away. It was something to remember."

What happened in the second leg remains one of the great nights in United's history. Sir Matt Busby's team went to Lisbon, where kickoff was held up by 15 minutes so that Eusebio could be presented with the 1965 European Player of the Year award.

"They knew they'd have a game on their hands against United and they delayed the kickoff because they wanted to get the crowd at it," says Crerand. "The same crowd had been holding five fingers up at our coach and beating on the windows as we edged towards the stadium. They thought they were going to beat us 5-0. That wound me up even more. 'Not a chance in hell,' I thought.

"The other lads were nervous," Crerand continues. "Me? I wasn't bothered at all and kicked a football around the dressing room as we waited. There was a mirror running the full length of the dressing room and I accidentally hit the ball against it, spreading shattered glass everywhere. Everyone went quiet, then Denis Law shouted: 'You crazy b------. That's it; we don't stand a chance now.' Many of the other lads agreed. But I wasn't superstitious."

Busby told his players to keep the game tight and keep possession until the crowd had quietened down.

"George Best chose to completely ignore this advice," says Crerand. "He ran amok from start to finish and absolutely slaughtered Benfica. Pulverised them. He scored his first in the sixth minute, his second in the 12th. Two minutes later he set up John Connelly to make it three. We were so dominant that we had two further efforts disallowed in the first half. George was 19 and he had just stunned 75,000 fans."

Benfica's home record had been 18 wins and a draw from 19 games played, with no defeats. On that March night, though, the best they could do was get one goal back.

"We'd done them and they knew it," says Crerand, who got United's fourth goal, 10 minutes from time.

Bobby Charlton made it 5-1 and cushions rained down from the stands as the home fans showed their displeasure. A couple of supporters ran onto the pitch at the end and tried to jostle the United players.

"I've been more scared sitting at home in my slippers with a cup of tea," says Crerand. "It was the greatest Manchester United performance I have ever been part of."

Crerand recalls being taken out in Lisbon after the match by Benfica players and he was in the United line-up again, two years later, for the most famous game between the clubs: The 1968 European Cup final, which United won 4-1 in extra time.

After that, the two giants of European football didn't meet competitively for 37 years, though they played a 1992 friendly for Eusebio's testimonial, a game that saw Eric Cantona first play in a United shirt.

Paired together in the 2005-06 Champions League, it seemed to be business as usual when United won 2-1 win at Old Trafford on Matchday 2, but Sir Alex Ferguson's side then went three games without a win and a 2-1 defeat in Lisbon saw them eliminated at the group stage for the first time in 11 years.

The English press smelled blood and sent their top writers to watch United get humiliated at a time when United fans were unhappy after a poor year, in which the club had lost the FA Cup final on penalties to Arsenal and been controversially taken over by the Glazer family.

"This club has always risen from difficult situations and will do so again," said an ashen-faced Ferguson.

He was right. The following season United won home and away against Benfica -- Louis Saha scored in both games -- on the way to the semifinals. The season after that, Ferguson's men were European champions.

But Benfica proved a thorn in United's side again in the 2011-12 group stage. Both matches were draws and the dropped points meant Ferguson's side finished third and were relegated to the Europa League; another disappointment.

Among the current Old Trafford squad, Victor Lindelof was signed from Benfica and Nemanja Matic played for the reigning Portuguese champions. Meanwhile, Jose Mourinho also managed at the Lisbon club and felt that he and his players were treated very respectfully in Lisbon two weeks ago, when United won 1-0.

Which is more than he felt when managing there in 2000; his first solo job only lasted a few months, despite good results. How Benfica must have regretted as he went on to have a stellar career, one that United fans hope continues with a deep run in this season's Champions League.