Apart from the European Championships for teams in mid-February, Antonsen had not played a tournament since the Indonesia Masters in January, and he also had difficulty settling into his game at the beginning of the match.
Watanabe gained the upper hand and took an 11-6 lead against an overly defensive Antonsen.
The short break did not help the Dane, who did not find the rhythm at all in his game in the first set. He fell behind 7-15 and virtually gave up during the last points of the set.
Antonsen had a better beginning to the second set and won the first two points, but then again lost his grip on the game.
Watanabe made virtually no mistakes, but he was also invited to take the initiative, and by winning eight out of the next nine points, the Japanese made it 8-3.
It looked like an early Danish exit, but down 4-10, Antonsen began to set the pace and play more aggressively, which allowed him to restore parity at 10-10.
But Antonsen again relinquished the initiative after the short break, and Watanabe was quick to take advantage of it. The Japanese grabbed six successive points to make it 18-11.
Antonsen exuded no faith in himself, and soon after, he was finished in Birmingham.