Novak Djokovic hits a backhand to Tennys Sandgren at the Western & Southern Open on Tuesday in New York City. Photo: VCG

World No.1 Novak Djokovic won his 20th straight match of 2020 with a hard-fought 6-2, 6-4 victory over Tennys Sandgren at the Western & Southern Open in New York.

Djokovic, who captured the Australian Open in February, needed six match points to tough out the straight-set victory and reach the quarterfinals of the hard-court tournament.

Djokovic breezed through the final game by winning four straight points, closing it out with a cross-court forehand winner to take it in 88 minutes on Tuesday.

"All in all it was a great performance," said Djokovic. "I felt better and played better than last night. I am going in a great direction."

American Sandgren, who is ranked 55th in the world, made the Serb work for the victory as he survived five match points in the ninth game of the second set.

Djokovic advances to the quarterfinals where he will play German Jan-Lennard Struff, who defeated seventh seed David Goffin 6-4, 3-6, 6-4.

This week's no-spectator event is a tuneup for the US Open which begins on ­August 31 on the same New York courts. This tournament was moved from Cincinnati to establish a quarantine bubble in New York for the upcoming Grand Slam.

Djokovic jokingly celebrated his win Tuesday by thanking all four sides of the empty stadium. Djokovic had pulled out of doubles on Sunday with pain in his neck, but did not appear to be bothered by the injury against Sandgren.

In women's action, fourth-seeded Naomi Osaka rolled over Dayana Yastremska in straight sets 6-3, 6-1 in the third round.

Osaka, who is one of two top-10 players to reach the third round, clinched the victory when Yastremska was called for a foot fault while serving on match point.

Japan's Osaka blasted eight aces, won 83 percent of her first-serve points and broke Yastremska's serve four times.

Osaka moves to the quarterfinals where she will face Anett Kontaveit, who defeated Marie Bouzkova 6-3, 6-3.

Yastremska, who at 20 was the youngest player left in the women's draw, made six ­double faults and won just 38 percent of her second serves.

In other matches Tuesday, Johanna Konta edged out former winner Vera Zvonareva 6-4, 6-2 to book her spot in the quarterfinals.

An out-of-gas Serena Williams crashed out, losing in three sets to 13th-seeded Maria Sakkari 5-7, 7-6 (7/5), 6-1. 

The 23-time Grand Slam winner finished with a whimper at the end of the two-hour, 17-minute match as Sakkari clinched the victory with a backhand down the line that a dejected Williams just watched without making an effort to move. At other points in the match, Williams flung her racket into the spectator-less stands and rebuked the chair umpire for slapping her with a time violation.