Borna Coric saved three match points before beating No.2 seed Pablo Carreno Busta, as the upsets continued at the Qatar Open on Tuesday.
The 48th-ranked Coric triumphed 5-7 6-2 7-6 (10-8), after saving three match points in the third set tiebreaker before taking advantage of his lone match point when Carreno Busta netted a forehand to secure his place in the second round.
Coric successfully broke Carreno Busta's serve when the Spaniard was serving for the match at 5-4 and 6-5 in the third set.
The Croat won 27 of the 33 points he played at the net, and had 44 overall winners in the 2 hour, 42 minute match.
"I didn't know what to expect," Coric said. "It's the first match of the year. It's always maybe a little bit tricky, but I can say from the beginning that I was playing very good.
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"In the off-season, I was working a lot on the net. I'm very happy with that stat."
The 10th-ranked Busta has won only one of nine official tour matches he's played since reaching the US Open semi-finals, where he lost to Kevin Anderson.
His defeat wasn't the only surprise on the day.
Third-seeded Tomas Berdych of the Czech Republic and fourth-seeded Albert Ramos-Vinolas of Spain were also upset in the first round.
Berdych lost 6-4 1-6 7-6 (7-4) to Jan-Lennard Struff of Germany, while Ramos-Vinolas was beaten 2-6 7-5 7-5 by Argentinian Guido Pella.
"Pretty tough match," the 53rd-ranked Struff said. "Very close. I know Tomas, he beat me in French Open. So I knew what to expect in the game.
"I played very aggressive in the tiebreaker and I was very happy after that one," Struff added.
Eighth-seeded Feliciano Lopez also advanced with a 6-4 5-7 7-6 (7-3) win over fellow Spaniard Guillermo Garcia-Lopez.
