New Zealander Ben Botica kicked 12 points to send Castres to third in the French Top 14 in Saturday's 15-9 home win over Stade Francais.
Botica's efforts extended the five-time champions' winning league record which stretches back to December 2020.
Ex-New Zealand centre Ngani Laumape started for the Parisians but was unable to inspire a late comeback in heavy rain.
After five games over the past two weekends were postponed due to Covid-19 cases, two matches this weekend were put on hold for the same reason.
South Africa's Cheslin Kolbe will have to wait for his Toulon Top 14 debut as they were scheduled to host La Rochelle while champions Toulouse were set to welcome Montpellier on Sunday.
The five fixtures going ahead were the final games to be played before players could be blocked from featuring after January 15 if the French upper house of Parliament passes a bill requiring a full course of vaccination to enjoy basic parts of life, such as entering sporting stadia.
Stade Pierre-Fabre had just 5,000 fans present, of its 12,500 capacity, with new regulations in force in France on spectators at outdoor and indoor events.
The hosts led 6-3 at the break after a turgid affair in heavy conditions as full-back Julien Dumora added to Botica's early penalty.
Joris Segonds kicked the Parisians only points of the half.
Botica, 32, the son of former dual-code New Zealand international Fran Botica, slotted three further penalties to make it 15-6 quarter of an hour after the interval.
- Latu sin-binned -
Wallaby hooker Tolu Latu was shown his Top 14 leading fourth yellow card of the season with 25 minutes left for making contact with Georgia prop Levan Chilachava's face.
In the closing quarter neither side looked to play with ball in hand as Castres head coach Pierre-Henry Broncan encouraged his team, including playmaker Botica, to kick the ball as much as possible.
Stade Francais had two attacking scrums in the final two minutes as the hosts' half-back Rory Kockott was shown a late yellow card but the 14-time winners were unable to claim victory and end the home side's impressive run on their own turf.
Later, leaders Bordeaux-Begles head to struggling Brive and former New Zealand fly-half Lima Sopoaga starts as Lyon host Pau.
There's a repeat of last season's second-tier promotion final as bottom-side Biarritz welcome 13th-placed Perpignan.
In the final game of the day, and the weekend, Clermont go to Racing 92 without their unvaccinated Japan full-back Kotaro Matsushima, in front of 2,000 supporters with the Parisians' ground an indoor arena.
Next weekend, Top 14 clubs are set to feature in European competitions after tournament organisers said on Friday the games would go ahead despite strict travel rules in place between France and Britain.