MONTREAL (AP) _ Howard Grant was virtually unknown both inside and outside of Canada when he entered the world boxing championships last month.

But because the 20-year-old Montreal welterweight won a silver medal, he's become the boxer everybody wants to beat at this month's Commonwealth Games in Edinburgh.

He had entered the world championships with the idea of giving it his best shot.

Grant proved effective enough to confound World Cup champion Kim Ki-taek of South Korea in the semifinal and earn a shot at the gold medal against Vasily Shishov of the Soviet Union.

''When I got in the ring to fight the Korean I thought this was it,'' said Grant. ''I knew he was a hard hitter, but I also had nothing to lose.''

The Korean was frustrated by Grant's ambidextrous style, just as Grant was to be frustrated by the Soviet boxer's disciplined performance in the later bout.

''I kept trying to sucker him and set him up, but the Russian wouldn't bite,'' said Grant. ''He just stayed outside, kept moving away, and didn't budge an inch from his pattern.''

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MONTREAL (AP) - Martina Navratilova, the No. 1 ranked women's tennis player in the world, has withdrawn from the Player's Challenge tournament here Aug. 2-10 so she can play in her native Czechoslovakia, organizers have announced.

Tournament director John Beddington said Wednesday Navratilova will represent the United States team in the Federation Cup match against Czechoslovakia.

''For the first time since she fled Czechoslovakia 10 years ago, Martina has a chance to go home,'' Beddington said. ''And under the circumstances, we believed she had a legitimate excuse to withdraw.''

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NEW YORK (AP) - Former New York Yankees Manager Billy Martin and ex- outfielder Bobby Murcer will serve as guest ''video DJ's'' on MTV next Tuesday, the music cable channel says.

Martin, a color analyst for the Yankees on television, and Murcer, the Yankees' color analyst on radio, will host two one-hour segments on Music Television from 7-8 p.m. EDT and 10-11 p.m. EDT.

The segments will be broadcast one week prior to major league baseball's 57th annual All-Star game on July 15 in Houston. Included in their one-hour show will be the baseball oriented videos ''Centerfield'' by John Fogerty and ''Glory Days'' by Bruce Springsteen.

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LAKEVILLE, Conn. (AP) - Paul Newman, who was scheduled to compete in Saturday's Kendall Nationals car race at Lime Rock, has withdrawn due to torn ligaments in his right wrist, a spokeswoman for Lime Rock Park said.

Newman was injured in a California race several weeks ago and has been advised by his doctor to rest, the spokeswoman said Wednesday.

The 61-year-old actor will be replaced by racing teammate Jim Fitzgerald driving a Nissan 300ZX sports car.