He's on 27 wins. The open era record is 50. He would need to win every match to the end of the season. Very slim chance.
Muster has the record after 1990 and that is a realistic goal for Fed. It is hard to compare undefeated stretches before 1990 since the TMS events have provided us fans with more matches with the best players playing against each other. It's a lot harder to go a long stretch without having to play a long list of top ranked players. Fed has laready played Hewitt twice, Roddick twice, Agassi, and Safin in this stretch.