One Saturday night last year, Phil Tulley was sitting at home in Monticello, N.Y., watching a telecast of the races at Roosevelt Raceway and was immediately smitten.

"Believe it or not, I was watching him on television and I was so impressed with the way the horse ran that I wanted to buy him," Tulley recalled yesterday of his first encounter with Mostest Yankee.

Time passed and Tulley had lunch with Jeff Mallet, the colt's trainer and son of Sid Mallet, who bought Mostest Yankee in 1978 for $8,500 from Charlie Keller, former New York Yankee star turned breeder in Fredericksburg.

"Jeff said his father wasn't interested in selling a whole horse, but would consider (selling) half and that was fine with me," said Tulley, himself the owner of the Woodstock Stud Inc. breeding farm.

So Tulley, who later sold 50 percent of his share to three friends, bought-"for a six-figure price"-half of Mostest Yankee, the heavy favorite in Saturday night's 25th running of the $75,000 William E. Miller Memorial Pace at Rosecroft Raceway.

Last year, Mostest Yankee won $110,863, winning 11 of 21 starts. This year, the bay colt has finished first in three of four paces, earning approximately $22,000.

As a 2-year-old last year, Mostest Yankee set the world's record on a half-mile track with a 1:58 4/5 mile at Roosevelt. His best time on a mile track is 1:56 4/5 at the Hoosier Futurity in Indianapolis.

The U.S. Trotting Association ranks the colt sixth nationality.

"Slowly, but surely, he's rounding into top form," said Tulley, who bred and sold Tijuana Taxi, also entered in the Miller Pace and "an awful good horse."

A striking feature about Mostest Yankee, Tulley said, "is he's so small when you see him on a race track. It's sort of like a David-and-Goliath confrontation sometimes. But he's got an awful lot of heart and desire and that makes up for his being small."

Mostest Yankee's performance Saturday will probably determine whether he is entered in the $750,000 Meadowlands Pace and the $307,000 Cane Pace, one of harness-racing's triple crown events, Tulley said.

Tulley adds that Mostest Yankee, who will be driven by Wayne Smullin, is "sound, sharp and good . . . I do think he's capable of beating any 3-year-old in the country, with the possible exception of Sonsan."