Toulouse's lead in the Top 14 table is down to a solitary point after Clermont Auvergne thrashed Grenoble. Lyon, meanwhile, lost at home.
Clermont Auvergne closed the gap on Toulouse at the Top 14 summit in style by thrashing Grenoble on Saturday – but high-flying Lyon surprisingly slipped up at home to Castres.
George Moala scored two of his side's seven tries as Clermont ran out 52-17 winners at Parc des Sports Marcel Michelin, with the landslide result moving them to within a point of the leaders.
However, Toulouse are yet to play in round 18 of the competition as they head to Stade Aime Giral to take on bottom club Perpignan on Sunday.
Taylor Paris and Kevin Firmin scored first-half tries as Castres upset Lyon, triumphing 23-15 to move up to fifth in the standings.
Jonathan Wisniewski kicked five penalties in a losing cause, but Lyon remain in third place thanks to nearest rivals La Rochelle going down 50-14 to Racing 92.
Teddy Thomas scored a hat-trick of tries in the resounding win while Finn Russell - who missed Scotland's Six Nations clash with France last weekend due to concussion - crossed the line once and also kicked a pair of conversions on his return to action.
Pau opened an eight-point gap over Agen after prevailing in the battle between 11th and 12th in the table, Colin Slade converting all three of his side's tries and also knocking over two penalties in a 27-17 triumph.
In Saturday's other top-flight fixture in France, Montpellier were too strong for Bordeaux-Begles, crossing for five tries in an emphatic 37-10 result.