U.S. Open champion Marin Cilic was shocked by fellow Croatian Ivo Karlovic in the opening rounds of the Shanghai Rolex Masters in China on Monday.

The veteran Karlovic, 35, had a late opportunity for a break on Cilic's serve late in the first set and took advantage to take the set but the 26-year-old Cilic got back in form in the second set to easily win it at 6-2.

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The third and deciding set was tense as both players doesn't want to give in on their serves but Karlovic found his mark in the tie-break to seal his victory 7-5 2-6 7(7)-6(2).

Karlovic converted a total of 20 service aces in the match compared to the nine of Cilic. He is currently the leader in aces this year, gathering a total of 885 aces in 50 matches and has the highest percentage in first serve points won at 84 percent.

"It's a gamble with him," Cilic said. "He was serving really well toward the end of the match. I couldn't get into his serve. It can happen with Ivo. It's always tricky."

Karlovic next opponent will be Yen-Hsun Lu of Chinese Taipei who defeated Spaniard Marcel Granollers in three sets.

The ninth seeded Cilic is currently in sixth place in the road to London and an early exit in Shanghai could jeopardize his position.

In an earlier match, Bulgarian star Grigor Dimitrov cruised through the second round after defeating Denis Istomin of Uzbekistan 6-3 6-3.

Dimitrov will face the winner between Frenchman Julien Benneteau and Ze Zang of China matchup.

The 10^th seed is in 11^th place in the shortlist for the ATP World Tour Finals slated November 10, 540 points behind Cilic. Since the event is part of the ATP Masters 1000, the winner will be awarded 1000 points and 600 points for being a finalist.

If the 23-year-old Dimitrov makes a grand performance in the tournament and reaches the finals, he could be the "dark horse" that would thwart Cilic's dream of making it into the prestigious season-ending event for the first time in his career.

TagsMarin Cilic, Grigor Dimitrov, Ivo Karlovic