Wawrinka missed the clay, grass and hardcourt season last year following surgery on his left foot and since returning in March has suffered a slew of first-round exits.

The Swiss former world number three played solid tennis but was unable to overcome hard court specialist Ruusuvuori, who smashed a forehand winner on match point to advance.

Next up for the Ruusuvuori is a second-round clash with Hubert Hurkacz (25), who the Finn defeated in Washington last week for the biggest win of his career.

Karen Khachanov (28) came out on the winning side in a sloppy three-set match against Francisco Cerundolo (23), where the players made a combined 110 unforced errors compared to just 45 winners.

Khachanov has played well at the tournament in the past, reaching the semi-finals in 2018 and 2019.

In other first-round action, American Jenson Brooksby (21) made quick work of Alexander Bublik (25) with a 6-2 6-3 win and Slovak Alex Molcan (24) outlasted Mackenzie McDonald (27) 7-6(1) 6-4.

Heavy rain delayed the start of the Masters 1000 tournament for hours and forced the postponement of Andy Murray's (35) highly anticipated first-round match against Taylor Fritz (24) to Tuesday, August 9th.

Follow all the results from Montreal as they unfold on Flashscore.