NEW YORK -- Canadian tennis star Milos Raonic advanced to the second round of the U.S. Open with a straight sets win over Germany's Dustin Brown on Monday.

Raonic, of Thornhill, Ont., fired 15 aces to Brown's two and totalled 41 winners en route to the 7-5, 6-3, 6-4 victory.

Earlier Monday, Vancouver's Vasek Pospisil earned his first Grand Slam victory of they year, downing Slovakia's Jozef Kovalik in straight sets.

Raonic ran into some trouble in the first set. Brown broke him in the fifth game, then won his next service game to tie it at 3-3. The two exchanged service points until Raonic broke Brown for the 7-5 win.

Raonic, seeded fifth at the tournament, is having a career year. He played in a Grand Slam final for the first time at Wimbledon, becoming the first Canadian man to reach a singles championship match at a Grand Slam.

Pospisil fired seven aces and broke Kovalik five times in a 6-1, 6-3, 6-3 victory.

Pospisil will face South Africa's Kevin Anderson in the next round.

It was an essential victory for Pospisil as he tries to turn his singles game around after falling from 39th at the start of the year to 123rd heading into the last major of the season.

He has a 7-21 singles record this year including first-round losses at the Australian Open, French Open and Wimbledon.

He has fared much better in doubles, where he is ranked 14th and reached the bronze-medal match at the Rio Olympics with Toronto's Daniel Nestor.

Eugenie Bouchard of Westmount, Que., plays her first-round singles match on Tuesday against Katerina Siniakova of the Czech Republic.