World number one Iga Swiatek was beaten in three sets by Jessica Pegula in the semi-finals of the Omnium Banque Nationale in Montreal.
Swiatek, who has topped the women's rankings for 72 weeks, lost out to world number three Pegula 6-2 6-7 (4) 6-4 after two-and-a-half hours.
Poland's Swiatek had led 4-2 in the final set after fighting back from the brink before her American opponent won the last four games.
Pegula had served for the match at 5-4 up in the second set and had led 4-2 in the tie-break before Swiatek hit back to force a deciding set.
