SIMPLE PAST
“I met her while I was a professional tennis player.”
If playing tennis was just a hobby why mention it in the first place? There are dozens of things someone might be doing at any given time. A more likely use of the Past Continuous would be to refer to one's residence.
PAST CONTINUOUS
"I was living in Missouri when we met for the first time.”
This suggests that the speaker no longer lives in Missouri. If the OP wants to emphasise the length of time they could say:
PAST PERFECT CONTINUOUS
I had been living in Missouri for ten months/years when I met my future wife.
In this instance, the speaker had lived in Missouri for ten months/years before they met. We don't know enough to surmise whether the speaker is still living there.
Likewise the OP's sentence “When I met her, I played tennis for ten years” should be:
When I met her, I had been playing tennis for ten years
Elsewhere in a related question, the OP asks
I just want to know if it's okay to say “While I was working 10 years I bought a house”? Yes or no that's it.
- I was working in Germany when I bought a house.
- I bought a house while I was working in Germany
There is no need to specify the duration, the act of purchasing a home occurred during that specific period–working in Germany.
This is because we don't normally use the PRESENT CONTINUOUS to say how long an action lasts.
- "I am working in Germany for ten years” NO
Instead, we'd use the PRESENT PERFECT or the PRESENT PERFECT CONTINUOUS
- I have lived / been living in Germany for ten years. YES