Canadian star Milos Raonic is out of the Australian Open, bowing out in a four-set loss to Slovakia's Lukas Lacko 6-7 (5-7), 7-5, 6-4, 7-6 (7-4) at Melbourne Park.
Raonic, an Australian Open semi-finalist two years ago, won a tie-break to go one set up, only to lose three on the trot to Lacko, ranked some 63 places below him.
The 22nd-seeded Raonic, who reached a career-high ranking of number three in 2016, sent down 36 aces to Lacko's 19, but Lacko ground out the crucial points that mattered in a close-run contest.
Another men's seed also fell as Fernando Verdasco axed Roberto Bautista Agut in a straight-sets win in an all-Spanish affair.
World number 40 Verdasco conjured up an early exit for the 20th-seeded Bautista Agut in a 6-1, 7-5, 7-5 victory.
Alexander Zverev was among the men's seeds to progress to the second round.
The fourth-seeded German claimed the scalp of Thomas Fabbiano in straight sets, 6-1, 7-6 (7-5), 7-5 on Rod Laver Arena.
Big-serving American Sam Querrey made short work of his opening-round match, thundering down 28 aces as he defeated Spaniard Feliciano Lopez 6-3, 6-4, 6-2.
The number 13 seed did not face a single break point as he overpowered Lopez, winning 85 per cent of points on his first serve.
