Kei Nishikori suffered a morale-sapping defeat ahead of his last-four meeting with Novak Djokovic at the ATP World Tour Finals, as Marin Cilic finally broke his duck at the O2 Arena.
Friday's last round-robin match had nothing riding on it besides ranking points and pride after Andy Murray's 6-4 6-2 triumph over Stan Wawrinka earlier in the day.
Wawrinka's defeat guaranteed Nishikori a place in the semi-finals and the Japanese initially looked set to celebrate with a routine win.
However, after taking the opening set with a degree of comfort, Nishikori's level dropped alarmingly and Cilic took full advantage to prevail 3-6 6-2 6-3 and record his first victory at this tournament at the sixth time of asking.
Cilic will move up to a career-high position of sixth in the world rankings as a result of his success.
An error-strewn second-set performance from Nishikori allowed Cilic back into the contest, but the Croatian produced some wonderful play in the decider - a backhand return winner to earn a crucial break of serve at 2-2 among the highlights.
Cilic  bt Nishikori  3-6 6-2 6-3
Cilic - 16
Nishikori - 1
BREAK POINTS WON
Cilic - 4/5
Nishikori - 2/3
FIRST SERVE PERCENTAGE
Cilic - 55
Nishikori - 64
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Cilic - 81/46
Nishikori - 64/44
Cilic - 82
Nishikori - 69