Manchester City had just made Premier League history when Pep Guardiola admitted he was unsure precisely how good they are.
"We will see what is our level when we face the last champions," he said, when Saturday's 5-0 win over Crystal Palace meant they had scored at least five goals in three consecutive league games.
Chelsea's subsequent Champions League victory over Atletico Madrid, in perhaps the finest away performance by an English club in Europe for years, indicates how high their own level is.
And, as league leaders face defending champions, Guardiola is entitled to wonder what City's superb start means.
They have won six games in September by an aggregate score of 24-1, but they began last season brilliantly too. Then their first defeat came on the corresponding weekend, in the capital and to title contenders, at Tottenham.
City have beaten Liverpool, but four of their five goals came after Sadio Mane's first-half red card. They have destroyed the rest, but can they beat the best?
It is easy to see why Guardiola deems Chelsea the acid test. City played much the better football for most of last December's meeting, but were caught on the counter-attack and lost 3-1 as Sergio Aguero and Fernandinho were sent off at the Etihad Stadium.
Chelsea were more clinical and ruthless: better in the boxes, in Guardiola's view, though not the better footballing side.
Yet City look less potent without the injured Aguero and, while they have the division's joint-best defensive record alongside Manchester United, their resources are stretched at the back. Vincent Kompany may be out. Benjamin Mendy is.
The chances are that Antonio Conte will look to expose whichever makeshift left-back, whether Fabian Delph or Danilo, Guardiola picks. His skills as a strategist were apparent in Madrid, a move to 3-5-2 reaping dividends.
The Italian tactician may look to stifle, keeping men behind the ball and breaking quickly. It is a safe assumption that City will have more of the ball.
The differing philosophies of two midfielders-turned-managers should be seen in the centre of the park: Conte will charge N'Golo Kante and Tiemoue Bakayoko with subduing Guardiola's playmakers David Silva and Kevin de Bruyne.
Chelsea may bring in a more defensive right-back, in Davide Zappacosta, for Victor Moses to mark the electric Leroy Sane.
The only opponents to halt Chelsea during a run of seven wins in eight games were Arsenal, who adopted a cautious approach and fielded a back three. Guardiola may do the latter but is unlikely to do the former. He is too ambitious.
But one who has been hired to be the best has only three wins against England's top seven clubs with City. After a £200 million (S$364 million) overhaul, this should show if City really are at Chelsea's level. Or even above it.
CHELSEA V MANCHESTER CITY
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