XPath - how do you select the child elements of a node?

I have an XmlDocument containing a XHTML table. I'd like to loop through it to process the table cells one row at a time, but the code below is returning all the cells in the nested loop instead of just those for the current row:

XmlNodeList tableRows = xdoc.SelectNodes("//tr");
foreach (XmlElement tableRow in tableRows)
{
    XmlNodeList tableCells = tableRow.SelectNodes("//td");
    foreach (XmlElement tableCell in tableCells)
    {
        // this loops through all the table cells in the XmlDocument,
        // instead of just the table cells in the current row
    }
}

What am I doing wrong? Thanks