Bordeaux Begles will have the chance to move into first place in Top 14 after leaders Lyon slipped up at Castres, while Handre Pollard had a difficult debut for Montpellier.
Lyon suffered their second successive league defeat as they went down 29-12 on Saturday, meaning Bordeaux will be top should they overcome La Rochelle on Sunday.
Depleted to 14 men twice in the first half, Lyon were 12-3 down by the interval, with Castres – who had two men sent to the sin-bin themselves after the break – scoring tries from Geoffrey Palis and Thomas Combezou before running in another two in the second half, Rory Kockott and Jody Jenneker touching down, to wrap up the points.
Montpellier failed to close the gap on the top two, as they were edged out 29-25 by Racing 92.
It was a frustrating day for debutant Pollard, who played a pivotal role in South Africa's Rugby World Cup triumph but has been sidelined with a facial injury, as the fly-half missed two penalties and a conversion.
#BoxingDaysRugby— TOP 14 Rugby (@top14rugby) December 21, 2019
Premier match de TOP 14 avec le @MHR_officiel pour le champion du monde sud africain Handré Pollard
Toulouse took advantage of Montpellier's defeat by beating struggling Agen 13-8 to move into third place with what was a third straight win.
Elsewhere, Brive and Bayonne shared the points in a 6-6 draw in Saturday's other fixture.