The South African opener made 115 not out and shared in an unbroken stand of 206 with Mark Ramprakash to take the visitors into a lead of 207 with nine second-innings wickets intact. Batting conditions tilted back in favour of batsmen with only two wickets falling in the day. Worcestershire added 161 before declaring at 251 for 5, and Middlesex progressed rapidly to 206 for 1 in 57 overs.
Initially the onus rested with Worcestershire, fifth in the table and hoping to close a 20-point gap on leaders Glamorgan. After crawling to 90 for 4 overnight, they were given new impetus by David Leatherdale's unbeaten 88.
Gavin Haynes (29) made only a limited contribution to a stand of 69, but Leatherdale hit 10 fours in topping 50 for the sixth time in his most successful summer for the county. Steve Rhodes was equally positive with a six off Phil Tufnell and five fours as he reached 59 not out in a partnership of 112 before skipper Tom Moody sacrificed bonus points in search of victory.
Middlesex lost Jason Pooley to the second ball - a 49th victim of the season for Alamgir Sheriyar - but the second wicket pair galloped along at nearly four runs an over. Kallis romped to 50 in 67 balls and reached three figures in 160 minutes with 20 fours, the last an edge through the slips of Sheriyar. Ramprakash played the secondary role for an unbeaten 72 in 163 balls.
Yorkshire's hopes of closing in on their first Championship since 1968 were lifted yesterday by the possible return of veteran seamer Peter Hartley for the title run-in.
The 37-year-old has been out of action since undergoing a hernia operation after playing in the NatWest quarter-final defeat against Glamorgan at Cardiff on 29 July, and he was not expected to figure again this season.
He has made a rapid recovery, however, and could return in tomorrow's AXA Life League Roses match against Lancashire at Old Trafford after coming successfully through a net session this week.
"I wanted to play for the second team at Bradford this week but it was felt that it might be too soon," said Hartley, who has claimed 23 Championship wickets this season.
"I am feeling good and so far there has been no sign of any reaction. It is possible that I will be available for Sunday, but if not, I would certainly hope to be ready for the Championship game against Worcestershire next week."
Hartley's return would be a major fillip for Yorkshire, who have been without their leading strike bowler, Darren Gough, since he was struck down with an inflamed left knee against Northamptonshire at the beginning of the month.Reuse content