Japan's world No. 109 Taro Daniel pulled off a major upset Sunday at the BNP Paribas Open by beating Serbia's Novak Djokovic in the second round.

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Though most of the crowd at Indian Wells came to see the former World No. 1 in his comeback following right elbow surgery, it was the 25-year-old Daniel who stole the show with a scrappy 7-6(3), 4-6, 6-1 victory.

"I can't believe I beat someone like (Djokovic) on center court like this, it's pretty crazy," Daniel said.

After a closely fought first two sets, Daniel said he sensed Djokovic, still clearly lacking match fitness, was tiring at the start of the third.

"I think I came out playing a little more aggressive than the other two sets and that made him keep making more mistakes. Because he obviously wasn't playing his best tennis, so I was able to take advantage of that," Daniel said.

Daniel, who reached the second round with a 6-3, 1-6, 6-1 victory over Britain's Cameron Norrie, had never beaten a top-20 opponent before Djokovic, and needed to win a pair of qualifiers to enter the main draw of the tournament.

"Getting through qualifying made me a bit more confident, which gave me the belief...to get through the tougher moments, even against someone like (Djokovic)," he said.

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Naomi Osaka continued her strong showing at the tournament on Sunday night, booking her ticket to the round of 16 with a 6-3, 6-3 win over Sachia Vickery of the United States. The world No. 44 served only one double fault on her way to wrapping up the match in 1 hour, 21 minutes.

The American came into the match after beating reigning Wimbledon champion and world No. 3 Garbine Muguruza of Spain in one of the upsets of the tournament so far, but surrendered too many easy points against her hard-hitting Japanese opponent.

Osaka, 20, will battle world No. 58 Maria Sakkari of Greece on Tuesday for a spot in the quarterfinals.

Daniel's third-round opponent is Argentina's Leonardo Mayer, who had been scheduled to play Japan's Kei Nishikori in the second round before the world No. 25 withdrew through illness. Mayer beat Nishikori's replacement Ruben Bemelmans of Belgium 6-4, 6-1.

Nishikori, who returned to competition in January after a five-month injury layoff, apologized for his withdrawal in a Twitter post on Sunday.

"I have been sick for over a week and tried everything I could to get better. Unfortunately today I am still not strong enough," he wrote.