Chelsea had little response as Celtic scored four in an eyebrow-raising defeat in front of a sparse crowd in Indiana.
Enzo Maresca’s side were comfortably second best at the Notre Dame Stadium, 2-0 down at half-time before conceding twice inside a minute late in the second half.
Christopher Nkunku scored a late consolation, finishing convincingly from the penalty spot in the 90th minute after Raheem Sterling was felled, but it did little to distract from an alarming defeat.
The Blues were unable to find their first win of their US tour, having drawn 2-2 with League One outfit Wrexham on Thursday in a heated match in California.
There was no ill will between Chelsea and Celtic in Indiana, but there was only one team at the races.
Celtic took the lead on 19 minutes when a cutely worked short corner was finished off by Matt O’Riley – the midfielder curling the ball beautifully into the far corner, past a helpless Robert Sanchez.
That lead was doubled 14 minutes later when O’Riley turned provider and squared for Kyogo Furuhashi to tap home into an empty net.
Raheem Sterling was one of three Chelsea substitutes to appear from the bench at half-time, and he added a threat to the Blues’ left flank, striking the post within a minute of the restart.
But it would be Celtic who again had the clinical edge in the second period, just as in the first, taking advantage of a defensive mix-up to score through Honduras’s Luis Palma.
Palma then put Michael Johnston through on goal. He duly slid under Sanchez to make it 4-0 Celtic. Nkunku soon after slammed home from the penalty spot, but Chelsea were already well beaten.
It caps off an excellent pre-season for Celtic, who had previously hammered DC United 4-0 and stunned Manchester City 4-3.
Chelsea, meanwhile, still have to face Mexican side Club America, Manchester City and Real Madrid, before returning to Stamford Bridge to host Serie A champions Inter Milan.
Maresca awaits his first win since replacing Mauricio Pochettino as Chelsea head coach.
