Updated

World No. 1 star Novak Djokovic round victory Monday at the 2011 U.S. Open, but American Mardy Wilfried Tsonga.

Djokovic, the reigning Wimbledon and Australian Open champion, was tested mightily in the first set before steadying the ship to beat 22nd-seeded Ukrainian shot-maker Alexandr Dolgopolov 7-6 (16-14), 6-4, 6-2 on a blustery day at the USTA Billie Jean King National Tennis Center.

Djokovic needed a tiebreak to sneak out the opening set, and that's exactly what he did with an ultra-exciting 30-point extra session. Dolgopolov jumped out to a 4-0 lead in the tiebreak, but the Serb fought back to force a lengthy breaker.

After Djokovic claimed the 76-minute high-quality first set, Dolgopolov seemed to lose his desire to stun the No. 1 star. The Serb cruised over the final two sets to secure a fifth straight trip into the U.S. Open quarterfinals.

"It was an interesting first set," Djokovic said. "I think it was exciting for the crowd to watch because it was very close. But I think it was game-wise an ugly first set because I wasn't happy with the way I played."

Djokovic tallied a mere 13 winners on Day 8, but Dolgopolov piled up 44 unforced errors to help the Serb's cause. Djokovic also broke the Ukrainian's serve five times, compared to only two breaks for the loser.

Dolgopolov was a surprise Aussie Open quarterfinalist back in January.

The 24-year-old Djokovic is now an amazing 61-2 this year, including a whopping nine titles. The Serbian slugger was last year's U.S. Open runner-up to Rafael Nadal and lost to Roger Federer in the 2007 final in Flushing.

Djokovic will meet good friend and fellow Serb Janko Tipsarevic in the quarters on Wednesday.

The 11th-seeded Tsonga rallied to beat Fish, seeded eighth, 6-4, 6-7 (5-7), 3-6, 6-4, 6-2. This is the first time Tsonga has reached the quarterfinals at the U.S. Open. His most successful Grand Slam result is the Australian Open, scene of his standout runner-up finish in 2008 against Djokovic.

Tsonga, who will face either Federer or Juan Monaco in the quarterfinals, broke Fish's serve in the second game of the fifth set and held a 5-1 lead. Fish actually saved three match points before Tsonga, who had 59 unforced errors, finally capped the lengthy affair that delayed the night session with an overhand slam.

The 20th-seeded Tipsarevic reached his first-ever Grand Slam quarterfinal by holding off former world No. 1 Juan Carlos Ferrero in four sets, 7-5, 6-7 (3-7), 7-5, 6-2, at Armstrong Stadium.

"I'm really happy, but I'm more happy that my goal of being top 20 at the end of the year is happening, you know," Tipsarevic said. "I don't know my ranking. On Monday I know it's gonna improve even more."

Tipsarevic is competing in his 31st career major tournament.

The 31-year-old Ferrero was the French Open champion and U.S. Open runner-up back in 2003.

The 27-year-old Tipsarevic has never titled on the ATP World Tour.