This spring, after he was soundly beaten in the Triple Crown races, Play Fellow seemed to be a plodder of very moderate talent.

But he has undergone a sudden transformation this summer, and when he rallied to win the 114th running of the Travers Stakes at Saratoga this afternoon, he became the leading contender for the country's 3-year-old championship.

What has brought about the great change in Play Fellow?

Very likely, it is jockey Pat Day.

The outstanding rider in the Midwest, Day took over the assignment on Play Fellow from Jean Cruguet three races ago, and the colt hasn't been beaten since. Day delivered another flawless performance today to score a 1 3/4-length victory over Slew o' Gold, with Hyperborean third and Deputed Testamony a disappointing fourth.

Because he always comes from far behind, Play Fellow is the type of horse who is prone to encounter traffic problems, which is what happened to him often when Cruguet was in the saddle. But Day had no difficulties when he won two rich races in Chicago this summer, and today he made it look easy again. "We were where we wanted to be the whole race," he said.

As is his custom, Play Fellow dropped back early, while the 20-to-1 shot Hyperborean sprinted to the lead. But the California invader wasn't getting the lead easily. The 2-to-1 favorite, Slew o' Gold, was stalking him, pressing the pace, and the two sped the first half-mile in 46 4/5 seconds and the three-quarters in 1:10 1/5. That is the kind of honest pace that a stretch runner like Play Fellow needs in front of him.

While the two leaders battled head-and-head around the turn, Play Fellow was still four or five lengths behind them, in third place. But Day wasn't worried.

"At the five-sixteenths pole," he said, "the leaders were driving, and my horse was still galloping."

Slew o' Gold got his head in front of Hyperborean in midstretch, but Play Fellow swung to the outside and wore them down inexorably. At the finish he was extending his margin with every stride.

Play Fellow covered the 1 1/4 miles in a respectable 2:01 over a track that had been muddy earlier in the day but was labeled "good" for the Travers. He paid $7.60 to his supporters in the crowd of 44,333.

Deputed Testamony was not able to gain appreciably in the stretch, even though the leaders were tiring in front of him. "The track was a little greasy and I don't think he ever got his stride," jockey Herb McCauley said.

More likely, Deputed Testamony is worn out after a long hard campaign. But his loss here probably cost him his chance for the 3-year-old title, which he could have clinched with a victory in the Travers.

Now that honor may go to Play Fellow. He whipped Kentucky Derby winner Sunny's Halo in Chicago; now he has trounced the Preakness winner, Deputed Testamony here. "I think," his trainer Harvey Vanier said, "that he deserves to be at the head of the class."