Novak Djokovic started his US Open campaign on Tuesday with a four-set victory against Danish teenager Holger Vitus Nodskov Rune. The Serbian tennis star registered a 6-1, 6-7 (5), 6-2, 6-1 win in the first round.

Djokovic has already won the Australian Open, French Open and Wimbledon this year. The top-seed is chasing history looking to complete the first calendar-year Grand Slam in men’s tennis since 1969.

Djokovic was scrathy against the 18-year-old Rune. The World no. 1 dropped the second set against the 145th ranked Rune who qualified at Flushing Meadows.

But all Djokovic needed to do was win, and he did. Every time Djokovic has played a Grand Slam match this season, whether on the hard courts of the Australian Open, the red clay of the French Open, the grass of Wimbledon, he has come out on top. The only challenge ahead of him are now the hard courts of Flushing Meadows.

After the match, Djokovic applauded his opponent for giving a tough fight and acknowledged that he wasn’t at his best.

“It wasn’t the best of my performances. But at the same time, he played well in the second set when it mattered,” Djokovic said.

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Djokovic is also looking to step ahead of his peers Roger Federer and Rafael Nadal with a 21st Grand Slam title. Both Federer and Nadal withdrew from the US Open this year due to injuries.

Earlier on Day Two at US Open, the top-seeded woman, Ash Barty, made a successful return to Flushing Meadows, the site of one of the two Grand Slam tournaments she has yet to win.

The biggest holdup for Barty during her 6-1, 7-6 (7) victory over 2010 US Open runner-up Vera Zvonareva came early in the first set, when there was a delay of more than five minutes because the Hawk-Eye Live electronic line-calling system went down when a TV camera wasn’t working.

(with AP inputs)