HARTFORD, CONN., JULY 24 -- For American tennis it was as dark a day as any in the country's long tennis history. For John McEnroe, in defeat, it was as gallant an effort as he has ever put forth.

McEnroe was not supposed to beat Boris Becker today. But when Tim Mayotte was stunned by Eric Jelen in the opening singles of this relegation match between the United States and West Germany, McEnroe had to beat Becker to give the United States a realistic chance to win the best-of-five series that continues with doubles Saturday and the last two singles Sunday.

McEnroe lost and West Germany has an almost insurmountable 2-0 lead. But McEnroe's courage in defeat will be long remembered by those who hung around the Hartford Civic Center for 6 hours 40 minutes tonight while he and Becker fought, scrapped, screamed and scratched until both looked like boxers who had pounded each other senseless for 15 rounds.

Becker finally won, 4-6, 15-13, 8-10, 6-2, 6-2, in the longest match in Davis Cup history -- surpassing the 6 hours 32 minutes played by McEnroe and Mats Wilander in 1981 -- wearing McEnroe down at last. McEnroe had not played a match for two months and, at 28, showed remarkable stamina until the very end.

But his valiant effort was wasted, partly because Becker refused to cave in despite McEnroe's tempestuous brilliance and partly because Mayotte had lost to Jelen, 6-8, 6-2, 1-6, 6-3, 6-2, in the opening singles, the one the United States had counted on winning. Jelen, ranked 68th in the world, played superb tennis in the clinches and Mayotte, ranked 14th, simply did not handle the pressure of playing at home, knowing he had to win.

Their match was, in truth, the important one because the United States had counted on beating Jelen twice in singles and on winning the doubles. "This was not a point we expected to win," Jelen said. "It makes our chances much better."

Coupled with Becker's epic victory, West Germany is firmly in control of the match. Ken Flach and Robert Seguso will be heavily favored to win the doubles and, if McEnroe can coax his body out of bed on Sunday, he should beat Jelen. But that would mean a Becker-Mayotte match would decide the victor and that is exactly the situation the United States was hoping to avoid.

"We're in a very bad position right now," McEnroe said. "Our only real hope is if they get in a tough doubles match and come back fatigued for the singles on Sunday. But right now it doesn't look very good for us."

It looks awful. Both these countries lost their first-round matches in Davis Cup play this year. The eight first-round losers each year face off in these relegation matches. The four losers are relegated for the following year to zone play. The four winners of zone matches -- now being conducted -- take their place in the main draw. If the United States loses, it will have to enter zone competition in 1988 to try and get back into the main draw for 1989.

There were 11,902 people in the Hartford Civic Center when Becker and McEnroe began play at 4:37 p.m. By the time Becker punched a backhand volley at 11:17 p.m. and threw his arms into the air in exhaustion and joy, less than half that many were left.

Those who stayed saw one of the most remarkable performances of McEnroe's extraordinary career. McEnroe had not played a match since losing in the first round of the French Open on May 26. In that time he skipped Wimbledon, acquired a diamond-stud for his left ear and agreed to play Davis Cup for the first time since the 1984 final in Sweden.

In McEnroe's absence, his country's Davis Cup fortunes have tumbled: a quarterfinal loss in 1985, a semifinal loss in 1986, a first-round loss to Paraguay this year creating the current predicament.

Things quickly got worse today when Mayotte could not hold a lead against Jelen. Once considered the future of German tennis, Jelen, 22, has had a run of bad luck and bad tennis in the last year, his ranking dropping from 30 to 68 in the 13 months since he upset Kevin Curren to open Wimbledon in 1986.

If Mayotte had played half as close to his peak as McEnroe came to his, he would have won easily. McEnroe has always found extra reservoirs of strength for Davis Cup -- hence his 35-8 singles record -- and today, with his team in trouble was no exception. He was pumped up and riding high from the start, breaking Becker in the second game of the match -- and he never stopped.

"Davis Cup," McEnroe said later, "tends to bring out the best and the worst in you."

The eloquence of that statement is hard to measure because it is so true, especially for McEnroe. This was the old McEnroe in every way: the tennis was brilliant, inventive, magical. The behavior was volcanic. He yelled at the German fans in the crowd, at chair umpire Claude Richard, at Becker, at himself, at tennis balls.

McEnroe won the first set with a break at love in the 10th game after starting the game by inciting the crowd by waving his arms and shaking his fists. "I thought I did a good job with that, getting them into the match," McEnroe said. "But after a while I only had so much energy. I couldn't do it anymore."

The second set was one of those classics only Davis Cup play can produce. It had remarkable tennis, unbelievable emotion, seven service breaks and controversy. Ultimately, it decided the match.

Becker broke for 2-1, McEnroe broke back. Becker broke for 3-2, McEnroe broke at love for 4-4, playing a game they should ship to the Hall of Fame right now -- four in-your-face winners. Becker broke for 8-7 and (surprise) McEnroe broke back with a running backhand he picked out of the air for a winner.

Becker, stunned, was hanging on. At 10-9, McEnroe had him 0-30. He hit a backhand, came in and had a backhand volley with the court open. He punched it an inch long. "That was the big point for me," Becker said. "He was playing so well I needed some kind of opening. After that I felt he might be a little shaky and it gave me confidence."

Instead of triple-set point, it was 15-30. Becker held from there. But moments later at 11-10, McEnroe did get to 0-40 when Becker double faulted. Then came the moment of the match. Becker boomed a serve. The line judge called it long. Richard, who had not overruled in the entire match, overruled and awarded Becker an ace.

McEnroe went crazy. He screamed at Richard. So did captain Tom Gorman. When West German captain Nikki Pilic walked over, McEnroe screamed at him to sit down. Richard was firm. The overrule stood. Becker hit three more service winners before McEnroe recovered. He had two more set points, but Becker saved them.

Becker finally took the set, breaking McEnroe at 13-all when McEnroe netted an overhead.

But McEnroe hung in, saving five break points during the third set and finally winning it 10-8. McEnroe was yelling to the heavens now, riding on emotion. But the rules require a 15-minute break and that gave Becker time to regroup.

Becker roared out after the break and broke immediately, racing through the next two sets. McEnroe was drained.

"We respect each other's tennis," McEnroe said. "He's a hell of a player and a hell of a competitor."

Those words fit Becker perfectly. And today they most certainly fit McEnroe. "I will never forget this match," Becker said.

Nor will anyone else who saw it.