Ivo Karlovic has smashed a record 61 aces in his opening round win over Yen-Hsun Lu at the US Open on Tuesday night.

Karlovic broke the previous US Open record for most aces in match of 49, set by Yevgeny Kafelnikov in 1999.

The big serving Croatian won the match 4-6, 7-6 (4), 6-7 (4), 7-6 (5), 7-5.

Unsurprisingly, Karlovic won 90 per cent of points on first serve.

Post-match, the 37-year-old said he had no idea he had broken the record.

"I knew there was a lot of aces," Karlovic said.

"I was, like, hitting them easy. At one point, almost every serve was an ace.

"The adrenaline ... I was in it, as the match was getting on, I was getting better."

Karlovic has now hit 11,277 aces in his career, the most in ATP history and 1,146 more than second-place Goran Ivanisevic.