You don't know about Aim for the Ace then.
Manga 1973-1980, 18 volumes.
First anime 1973-1974
Second anime 1979-1980 + movie 1979
OVAs 1988-1990
Live Action TV Show 2004
The relevant info is here:
The manga was a hit, having sold approximately 15 million copies, which makes it one of the best-selling shōjo manga series of all time. Although the television series initially received low ratings, reruns were more positive received by fans, as well as the second series. The anime is considered a hit on Japanese television and though aimed toward schoolgirls it was popular among people of both sexes, and was also popular in Europe. In 2001, the anime magazine Animage ranked the 1979 television anime as the twenty-second of the Top 100 anime productions of all time. In 2005, TV Asahi conducted a "Top 100" online web poll and nationwide survey asking viewers for the best television anime series; Aim for the Ace! was placed fourteenth in the online poll and thirty-fourth in the survey. Responsible for a tennis boom among high school students in the 1970s, the series was still popular as of 2015 appearing in several online web polls of most influential sports anime.
The 1973 series was in fact cancelled halfway because of low TV ratings, but that changed as it became very popular with re-runs, leading to the full 1979 series. So the idea of "not very popular" is only true if you're living in the mid 1970s.