Taiyou no Shita de Warae (1999)

Blurb:
After seriously injuring an opponent in a boxing match-Sohei quits his dream of ever being a pro boxer. Ten years later, after a string of failed jobs and relationships, his friend Chika finally convinces him to get back into boxing. But where Sohei was once champ material, he is now 25 years old (over the hill in boxing years) and must play catch up with all the 18-year-olds at the gym. To make things worse, Chika’s been secretly in love with Sohei for years, and has about had it with waiting for Sohei to figure it out. With his upcoming pro test, his squabbles with the youngsters in the boxing club, and Chika’s increasingly bold advances, will Sohei finally find some direction and happiness in his life?

Source: www.anisearch.com/manga/2126