Germany's Alexander Zverev became just the third man to beat Roger Federer this year as the 20-year-old German won his fifth title of 2017 in Montreal on Sunday.
Zverev, ranked eighth, won 6-3 6-4 at the Rogers Cup to claim his second Masters 1000 title and extend his winning run to 10 matches.
Federer, 36, lost for just the third time in 38 matches this year, reports BBC.
The Swiss can still take the number one ranking if results go his way at the Cincinnati Open next week.
Either Federer or Rafael Nadal will replace Andy Murray at the top of the standings on 21 August.
Zverev gave further evidence that he is a contender for the biggest titles as he became only the second active player after Jo-Wilfried Tsonga to win two Masters titles, outside the 'Big Four' of Federer, Nadal, Novak Djokovic, and Andy Murray.
The German has drawn level with Federer on a Tour-leading five titles in 2017, following up last week's victory in Washington.
