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Rafael Nadal made a successful Olympic singles debut on Monday, the Spaniard albeit needing three sets to beat Italy's Potito Starace 6-2 3-6 6-2.
However, top seed Roger Federer had an easier time as his own bid for a first Olympic medal got underway with a 6-4 6-2 victory against Russian Dmitry Tursunov.
Nadal, the French Open and Wimbledon champion, broke twice to take the first set. However, a running forehand pass from Starace allowed his opponent to go 2-0 up in the second.
The Spaniard then squandered five break points, with Starace surviving three deuces at 4-2 before eventually levelling - and taking a set off Nadal for the first time in five meetings.
But the 27-year-old's inspired form deserted him in the deciding set, with Starace netting at 2-3 for the initial break and again on Nadal's first match point.
"I had a very good first set and I'm happy about that," said Nadal, who only played in the doubles event in Athens four years ago.
"Then I had a break against in the second set and I had a lot of opportunities to break back but I couldn't, so that was tough.
"He had more problems than me when he was serving so that gave me a lot of confidence. Finally I got the break and match."
Behind schedule
Meanwhile, after arriving on Centre Court 24 hours behind schedule because of Sunday's rain, Federer wasted little time in dismantling his opponent.
The Swiss star, who has recently suffered early exits in both the Toronto and Cincinnati Masters events, broke serve once to win the opening set and then moved up a gear to cruise through the second set.
Nadal will replace Federer as world number one next Monday.
Elsewhere, Nicolas Massu made a strong start to the defence of his title, the Chilean beating Steve Darcis of Belgium 6-4 7-5, while Argentina's David Nalbandian saw off China's Zeng Shaoxuan 6-2 6-1.
French 10th seed Gilles Simon went through against Robin Soderling, joining Mikhail Youzhny, Olivier Rochus, Thomas Johansson and Victor Hanescu in the second round.
Just nine matches were completed on the opening day as thunderstorms lashed the Olympic Green Tennis Centre. Some 65 are scheduled for Monday.