Withstanding Sauron

[Maéglin]

At the last the siege was so strait that Sauron himself came forth

Doesn't that actually imply that the siege of Barad-dur was so close with neither forces unable to defeat the other that Sauron himself had to enter the fray. I don't think Sauron counted on the might of Gil-galad and Elendil together. After all he was a shape-shifter. Couldn't he just have high-tailed it out of there in Vampiric form.