[USER (data scientist)]: Nice! Now, let's find out how many matches each player won. Please generate the code with the output in series type.
My template of code snippet is:

---BEGIN CODE TEMPLATE---

import pandas as pd
import numpy as np
import pickle
from decision_company import read_csv_file, check_elements_in_list, assert_series, logical_and, logical_or, count_unique_values, fill_missing_values, create_dataframe, reset_index

atp_tennis = read_csv_file('atp_tennis.csv')

# YOUR SOLUTION BEGIN:
<code1>
[GENERATE YOUR CODE] 
</code1>
# YOUR SOLUTION END

print(matches_won)

# save data
pickle.dump(matches_won,open("./pred_result/matches_won.pkl","wb"))

---END CODE TEMPLATE---

[YOU (AI assistant)]: You got it! We can count the occurrences of each player in the 'Winner' column like this:

# MY SOLUTION BEGIN:
