The defending champion United States defeated Czechoslovakia, 2-1, yesterday in the championship round of $500,000 World Team Cup tennis tournament. It was not easy.

A 6-3, 7-6 (7-3) doubles victory by Ken Flach and Robert Seguso over Ivan Lendl and Tomas Smid decided it after the teams split in the singles, Lendl beating John McEnroe before Jimmy Connors won a tough match from Miloslav Mecir at the Rochus Club in Du sseldorf, West Germany.

Lendl reversed a loss to McEnroe in last year's final by coming on to prevail in a duel of the world's top two current players, 6-7 (7-4), 7-6 (8-6), 6-3, in a 3 1/2-hour ordeal made more difficult by 113-degree temperatures on center court.

Connors evened the match by eluding the fast-improving Mecir, 6-3, 3-6, 7-5. Mecir, winning eight straight games from midway of the second set, was up, 4-0, in the third before evidently falling victim to a case of nerves.

Mecir began repeatedly double-faulting -- three times in the 11th game to fall behind.

"I don't know what happened down there," Connors said. "I've never had a guy serve underarm against me in my life. He got tight, and the tighter he got the looser I got." . . .

Mikael Pernfors of national champion and tournament host Georgia became collegiate men's tennis' first repeat singles champion in 21 years (since Dennis Ralston), mastering his teammate/roommate George Bezecny, 6-2, 6-3, in the NCAA singles final.

With Pernfors, a senior from Sweden, playing a Bulldog senior from Fort Lauderdale, Fla., it was an uncharacteristically quiet Athens crowd of 4,457. Georgia's fans, known for intimidating opponents, had no one to root against . . .

Bonnie Gadusek defeated top-seeded Manuela Maleeva, 6-2, 6-2, to win the $100,000 women's European Open in Lugano, Switzerland . . . Spain's Sergio Casal won the Florence International, defeating New Yorker Jimmy Arias, 3-6, 6-3, 6-2.