The countdown to the 42nd Masters Tournament proceeded smoothly yesterday as most of the 78 players preparing for the 1978 renewal practiced in balmy 80-degree weather for the start of competition Thursday.

Five-time champion Jack Nicklaus arrived in midafternoon and pronounced his game and the immaculate National course to be in excellent condition. Fairway "flyers", the bookmakers, promptly installed Nicklaus the 5 to 2 favorite to add another title to those he won in 1963, 1965, 1966, 1972, and 1975.

"I think my game is better now than it has ever been," said Nicklaus.

Four-time champion Arnold Palmer spent much of the day explaining why he keeps playing. "I still think I can win," Palmer declared, although he hasn't scored a major victory since the winter of 1973 and his successes here date to 1964, 1962, 1960 and 1958.

Three-time champion Sam Snead played 18 holes, something of a major triumph in itself for the 1949, 1952 and 1954 winner. A bone spur on his right heel has pained Snead this year, limiting his PGA Tour appearances to two and his finishes to one.

Two-time champion Gary Player, encouraged by his improved effort at Greensboro (tie for 19th), is even more enthusiastic about Welcome Boy, his 3-year-old colt that recently won the South African Derby at Johnnesburg "by seven lengths, all alone." Player won here in 1961 and 1974 and lost in '62 in a playoff with Palmer.

Defending champion Tom Watson spent much time on the practice tee and putting green. "I feel better about my driving," said Watson, who failed to make the cut in two of his last three tournaments.

While the past-Masters and the more current Masters prepared for this week's 72-hole grind, expected to be played in unseasonably warm weather, a young Castillian many observers point to as a future master of the Masters was talking with caddy master Freddy Bennett.

"Can you fix these for me?" Severiano Ballesteros asked, pointing to the grips on his clubs.

"Fix 'em after you had 66 with 'em on Sunday (to win the Greensboro Open)?" Bennett asked, not quite believing the request.

"Yes, they're loose," Ballesteros said. "I have the extra grips."

"Then I'll do 'em for you," Bennett said. "It's good you had 'em, cause I didn't. I don't see that (Mizuno) make too often."

Ballesteros will be 21 Sunday. The Spaniard has the dark, smooth features of a movie star and the style, on a golf course, of a matador.

"With me, it is not too usually dull," Ballesteros acknowledged. "It can be birdie-birdie or bogey-bogey-bogey, or even better or even worse. My play is still too much like it was last week (72-75-69-66). My driving and my putting are good, my irons not so good."

Driving and putting are what the Masters is all about. "His game fits this course," 1970 Masters champion Billy Casper said. "The young man is long, very long and he's a great putter. He could win here, and he could win here maybe more than once.

Bookmakers cut Ballesteros' price to 10 to 1, placing him in a cluster with Player, Lanny Wadkins, Jerry Pats, Andy North, Gil Morgan and Ray Floyd, the 1976 asters hero.

Watson, the PGA Tour's "player of the year" last season, is the second choice to Nicklaus, at 4 to 1, followed by Hubert Green and Tom Weiskopf, at 5 to 1.

Nicklaus, Watson and Green are the only double winners on the Tour this year. Nicklaus scored at Inverrary and in the Tournament Players Championship over Windy Sawgrass. Watson started smartly, taking the Tucson and the Crosby. Green won in Hawaii and the Heritage at Hilton Head.

Weiskopf is 5 to 1. The four-time Masters runner-up (1969, 1972, 1974, 1975) won at Doral in February and has impressed in practice rounds here, driving exceptionally long and straight.

Hale Irwin, 6 to 1, and Ben Crenshaw, 8 to 1, are the only other players listed in the elite group. Irwin has made the cut in 62 consecutive tournaments, a tour record. He was second to Green over Harbour Town. Crenshaw shared the 54-hole lead here last year.

The new face attracting the most attention is Ballesteros.

"I think, maybe, four 69s (276) should win this," he said. "Somebody will do that, or maybe a little better. All I'm confident is I'll do better than I first did (tie for 33rd last spring). But I'm not going to play in Europe. Three, four years from now I'll come here to play. I promise."

A 276 would be a considerably lower total than was required to capture the tournaments at Jacksonville, Hilton Head or Greensboro in recent weeks, but it would be the same score Watson posted in winning last year by two strokes over Nicklaus.

Washington's Lee Elder tied for 19th place in the 1977 Masters with 287 and is back for another try. Elder, the first black golfer is qualify for this event, has not won a tournament in the past year but his Masters finish earned him a place in the lineup, the first 24 including ties being invited back. Elder failed to make the cut in his historic 1975 appearance.