Thomas has 20 career NHL shutouts, five against the Senators.
On Thursday night in Boston, Thomas beat Toronto 2-0.
Boston opened the scoring early in the first period when Krejci took advantage of an Ottawa turnover. Chris Phillips' attempt to clear the puck caught Mike Fisher's stick, and Krejci caught Elliott off guard.
Ottawa appeared to tie it midway through the second on a goal by Daniel Alfredsson, but it was called back after it was ruled Fisher interfered with Thomas in the crease.
The Bruins led 3-0 after two period.
Seguin, with his third of the season, took a great cross-ice pass from Mark Recchi and caught Elliott moving side-to-side to beat him shortside on the power play at 13:55.
Ottawa looked to be asleep toward the end of the period, surrendering two breakaways in succession. Elliott stopped Nathan Horton's attempt, but Lucic made scored moments later.
Caron then scored on a breakaway in the third.
NOTES: Thomas has allowed only three goals in six games this season. ... Jason Spezza returned to the Ottawa lineup after missing four games because of a groin injury.