Manager Garry Monk labelled Swansea City's first-half performance against Chelsea as their "worst of the season" following a 5-0 Premier League defeat.
Swansea looked helpless in the opening 45 minutes at the Liberty Stadium as a ruthless Chelsea ran in four goals, albeit with the help of some questionable home defending.
Oscar and Diego Costa both took advantage of Swansea's shortcomings to fire Chelsea into a 4-0 lead at the break with a brace apiece, before Andre Schurrle added a late fifth for the league leaders.
Monk's men had improved after the break to keep the visitors quiet for most of the half, but the 35-year-old said it was too little too late.
"It was a horrible performance in the first 45 minutes, way off standards that we have set," he said. "It is very difficult to make mistakes continuously, coupled with fact that it is against Chelsea, which magnifies it more.
"We got punished every time we made a mistake and, ultimately, the first 45 minutes cost us the game. It was always an upward battle after that.
"We had to be perfect to stand a chance. We weren't, we were far from perfect.
"Everything we worked on, we didn't do. In the second half we were marginally better, but that first 45 minutes was our worst of the season by far.
"It's hard to explain the drop-off in performance. We are still in good place, but days like today are very hard to explain.
"They are good players, they just didn't show that today at all."
The defeat is Swansea's heaviest of the season and extends their winless run in the league to four matches.
