For a couple of unfortunate Americans, the Latin motto most applicable to this year's Italian Open tennis championshps was not the familiar "veni, vidi, vici" ("I came, I saw, I conquered"), but rather "veni, vidi, victus fui" -- "I came, I saw and I was conquered."

Gene Mayer and Eddie Dibbs, the No. 3 and 4 seeds, not only failed to reach their anticipated places in the semifinals at Ford Italico, but sustained injuries that will definitely keep Mayer out of the French Open, which begins Monday in Paris, and may eliminate Dibbs as well.

Mayer, who at age 25 has rather quietly climbed to No. 6 in the world rankings, severely sprained his right ankel in a second-round match against Austrailian Peter McNamara. He ran in to cover a drop shot, his foot stuck in the rain-dampened clay, and he tumbled over in obvious agony.

X-rays revealed no fracture, but the sprain was severe enough to require a cast, which Mayer will wear for three weeks. He withdrew from the French and returned home to the United States, but hopes to be able to play at Wimbledon, startng June 23.

Dibbs pulled a groin muscle while beating Spaniard Fernando Luna by the unlikely scores of 4-6, 6-0, 6-0, but aggravated it in his third-round match against Yannick Noah. Dibbs lost the first seven games, then defaulted. He received treatment and hopes to be ready for the French.

If Dibbs plays, the French will have 16 of the current top 20 men, including to top five: Bjorn Borg, John McEnroe, Jimmy Connors, Guillermo Vilas and Vitas Gerulaitis. Only Wimbledon and the U.S. Open have comparably strong draws.

Borg -- who did not play the Italian, but came to Rome for a press conference announcing his new, seven-year clothing contract with Fila, estimated at $3 million -- is the odds-on favorite to win the French. If he takes the singles title, it would be his third in a row and fifth overall, both records for the tournament regarded as the world clay-court championship and most physically demanding to win.

"You are dying to win those big tournaments, like the French and Wimbledon and U.S. Open, because they are the most important, every year," said Borg, who will celebrate his 24th birthday during the French. "You play two weeks, you know everybody is there and everything must work out perfectly."

As if Borg needed any help on clay, a surface on which most of his colleagues regard him as virtually unbeatable, he is in probably the easiest section of the French draw. He should encounter no trouble until the quarterfinals, where he will likely meet either Jose Higueras -- who has averaged just over one game a set against Borg in their last dozen meetings -- or Victor Pecci, who troubled him in last year's Paris final.

If the seedings hold true, the other quarterfinals in Paris would put 1977 champion Vilas against Harold Solomon, Dibbs (if he plays) against Conners, and Gerulaitis against McEnroe.Connors has the only really difficult first-round opponent: 1976 champ Adriano Panatta, who has played some splendid matches against him on various surfaces.

The women's draw in Paris is suffering for the second straight year from the disappointing absence of Martina Navratilova and Tracy Austin, now the world's two best players. If the seedings hold, the quarterfinals would be: defending champ Chris Evert Lloyd vs. Kathy Jordan; Virginia Wade's va. Hana Mandlikova; Virginia Ruziel vs. 1979 runner-up Wendy Turnbull, and Dianne Froholtz vs. Billie Jean King.

Rino Tommasi, the crack Italian tennis writer, statistician and oddsmaker, has prepared a form chart for the French, showing the results of the seeded men so far in 1980. Borg has compiled a record of 31-1 in six tournaments. The over-tennised McEnroe is 33-10 in 10 tournaments and suspect on clay. Connors is 31-8 in 10 tournaments and in good form. Vilas is 27-9 in nine tournaments and steady, while Gerulaitis is 18-9 in nine tournaments and streaky.

Betting on tennis is popular in Italy, but more on the outcome of individual matches than on tournament winners, as is the norm in Great Britian.

A friend of Tommasi's won 2 million lire -- about $2,300 -- Thursday when Tomas Smid of Czechoslovakia beat 16-year-old Frenchman Thierry Toulasne, who had upset defending champion Gerulaitis the previous day.

This humble handicapper was not so fortunate. Like Mayer and Dibbs, I came to Rome, I saw, and I was conquered. Veni, vidi, victus fui.

Victus. Phooey.