Sam Robson and Leus du Plooy shared a Middlesex record third-wicket stand of 252 as the home side took control of their Vitality County Championship clash with Leicestershire at Lord's.
Robson marked his return from a thumb injury with an accomplished 162, while Du Plooy smashed 131 from 191 balls - his second century since moving from Derbyshire in the winter - to help the Seaxes to 407 for eight.
That was a first-innings lead of 101, with a quartet of bowlers - Ben Mike, Scott Currie, Ben Green and Rehan Ahmed - all taking two wickets apiece for the Foxes.
However, with the first day completely washed out and Monday's weather forecast gloomy, a draw looks the most likely outcome. Having lost one wicket the previous evening, Leicestershire doubled their tally in the first over of the day when Max Holden drove outside off stump to Currie and was caught deep at slip without adding to his overnight score of 26.
But that proved to be the visitors' only success of the morning session as Robson and Du Plooy piled up runs with little cause for alarm.
Robson, who had already hit a couple of boundaries square of his pads, survived a scare on 39 when he edged Currie down the leg side and wicketkeeper Ben Cox, diving to his left, got a glove to the ball but could not get it to stick.
A clean leg-before shot from Tom Scriven took the opener past 50, while Du Plooy opted to give Green the charge, moving down the track to hoist a short delivery into the stands for six.
Rehan received similar treatment when he came on before lunch, with Du Plooy hitting the England leg-spinner for four and cutting the next ball for three to reach a half-century.
Middlesex upped the tempo after the break and Robson won the race to three figures with his partner, driving Scriven through midwicket for a three to reach the landmark from 198 balls.
Scriven, swinging the ball from the nursery end, was unlucky to remain wicketless - Du Plooy persuaded him to edge to Peter Handscomb at leg slip but the former Middlesex captain could not hold on.
Still 16 short of his hundred, Du Plooy took advantage of the dismissal to score a ton. The second new ball proved fruitful for the batsmen, who soon accelerated past Leicestershire's total of 306.
Robson took advantage of a trio of short deliveries from Mike to pull him for two fours and a six before tea, but the Foxes' bowlers finally began to make an impact in the final session.
Du Plooy was the first to go, bowled one behind by Currie, and Ryan Higgins was bowled swinging by Green, who also claimed the scalp of Robson, neatly caught at backward point to end his six-hour stay at the crease.
Nathan Fernandes kept the momentum going for Middlesex with a pugnacious 25-ball knock of 31, which included sixes off Green and Rehan before he was holed out to deep midwicket, while Toby Roland-Jones also fell in the pursuit of quick runs before bad light intervened.