London Lions suffered overtime defeat against Caledonia Gladiators in their BBL clash at the Copper Box Arena on Sunday.
The defending champions went down 100-94 and have a 15-3 record going into their next EuroCup contest this week.
Josh Sharma produced an early one-handed dunk for the hosts, but the visitors went on a 6-0 run to move 11-4 up.
Luke Nelson's three-point shot cut the gap to two, but Gladiators saw Trent Buttrick sink a triple from the corner to make it 24-13 at the first break.
Lions rallied in the second quarter, as Gabriel Olaseni and Bradley Kaboza got them back to within six.
And the sides traded threes, with Conor Morgan, Kaboza and Nelson on target for the hosts, before Sharma put them 34-33 up.
But just when it looked like they had the momentum, Lions saw their rivals go on a 14-2 tear to turn the game on its head once again.
A pair of free throws by Nelson closed out the half, reducing the deficit to single digits (47-38), and the same player answered with an and-one after Patrick Whelan's long three at the start of the second half.
Kaboza then boxed out the much taller Fahro Alihodzic and went coast-to-coast before dishing to Olaseni for an easy two, before Kareem Queeley's steal and dunk completed an 11-4 run to leave Lions 51-49 down.
Olaseni's dunk on a fast break put them back in front, but the Scottish side moved 59-54 up before Lions went on a 7-2 run to level it at 61-61.
Threes from Ian Dubose, Princeton Onwas, Jordan Taylor and Whelan left Caledonia 78-73 up, before Lions went on a 5-0 run to tie it once more.
Whelan's lay-up and Olaseni's pair of free throws kept it all square with less than a minute left, before Buttrick's massive three with 11 seconds to go.
Taylor sank an off-balance three on the buzzer to make it 83-83, then answered Lukas Palyza's triple at the start of overtime.
But Whelan's fifth trey put Caledonia 97-90 up and Palyza drained his third three of the extra period to bring up their century, before Taylor's free throw ended the scoring.
Lions head to Umana Reyer Venice on Wednesday, looking to improve their 7-2 record in Europe after a convincing 101-87 win over Wolves Vilnius in Lithuania last week, when Conor Morgan scored 36 points.
Megan Gustafson's 17 points led London's women to a 78-63 win over Rutronik Stars Keltern, improving their EuroCup record to 6-0, before scoring another 18 points in their 77-61 success over WBBL rivals Leicester on Sunday.
They host Nottingham Wildcats on December 10 (1.15pm), before heading to Turkey to take on Bursa Uludag four days later.
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