The ITF World Tennis Number (WTN) is a system introduced by the International Tennis Federation (ITF) to provide a more accurate and global ranking system for tennis players. The probabilities and expected wins are based on the assumption that WTN is a perfect predictor of match outcomes, which it isn't in reality due to many other factors like player form, strategy, and even luck.

What is the ITF WTN?

Global Rating System: The WTN aims to give players a single rating number that reflects their skill level accurately, regardless of age, gender, or the country they play in.
Range: The WTN is expressed in decimal form, with a range typically from 40 (beginner level) to below 1 (professional level). The lower the number, the better the player.

How Does It Work?

Match Results: Every match result contributes to a player's WTN. This includes matches from competitions, league games, or even practice matches if they are entered into the system.
Algorithm: 
    The system uses an algorithm that considers several factors:
        Opponent's WTN: The rating of the player you're competing against significantly impacts how your WTN changes. Beating a higher-rated player will boost your WTN more than beating a lower-rated one.
        Match Format: Whether it's a singles or doubles match, the format influences the rating change.
        Scoreline: The closeness of the match; a close win or loss affects your WTN differently than a straightforward set.
Dynamic Adjustments: Unlike traditional ranking systems where points are static for a period, the WTN can change after every match. This makes it very responsive to current form but also means it can fluctuate more than traditional rankings.
Accessibility: Players can see their WTN through the ITF's official platforms or through national tennis federations if they've enabled WTN tracking for their tournaments.
Privacy and Sharing: Players can choose to share their WTN or keep it private. This aspect is crucial for those who might not want their skill level widely known, especially for privacy or competitive reasons.
Integration: The WTN is designed to integrate with existing ranking systems but offers a more nuanced view of player ability, potentially complementing or even replacing traditional systems in some contexts for a more uniform global standard.

How Does It Work?

Match Results: Every match result contributes to a player's WTN. This includes matches from competitions, league games, or even practice matches if they are entered into the system.
Algorithm: 
    The system uses an algorithm that considers several factors:
        Opponent's WTN: The rating of the player you're competing against significantly impacts how your WTN changes. Beating a higher-rated player will boost your WTN more than beating a lower-rated one.
        Match Format: Whether it's a singles or doubles match, the format influences the rating change.
        Scoreline: The closeness of the match; a close win or loss affects your WTN differently than a straightforward set.
Dynamic Adjustments: Unlike traditional ranking systems where points are static for a period, the WTN can change after every match. This makes it very responsive to current form but also means it can fluctuate more than traditional rankings.
Accessibility: Players can see their WTN through the ITF's official platforms or through national tennis federations if they've enabled WTN tracking for their tournaments.
Privacy and Sharing: Players can choose to share their WTN or keep it private. This aspect is crucial for those who might not want their skill level widely known, especially for privacy or competitive reasons.
Integration: The WTN is designed to integrate with existing ranking systems but offers a more nuanced view of player ability, potentially complementing or even replacing traditional systems in some contexts for a more uniform global standard.