William Lewis was the strongest player in England in the 1820s. He and John Cochrane were crossing the channel to represent England against the top French players such as Deschapelles and Labourdonnais.
McDonnell knew he would have to compete against players of this level to achieve his full potential. Lewis, like many top masters, supplemented his winnings in stakes by coaching promising amateurs for a fee. He saw potential in McDonnell and mentored him.
McDonnell's game flourished under the tutelage of Lewis. His positional and tactical acumen were sharpened significantly. Inevitably he eventually overtook Lewis and became the better player. Lewis, fearing for his reputation, would not play him any more.