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Nadal took three sets to beat Italy's Potito Starace
Wimbledon and French Open champion Rafael Nadal had a three-set test in a first round win against Italy's Potito Starace in the Olympic men's singles.
The second seed needed a deciding third set to eventually beat the 71st-ranked Italian 6-2 3-6 6-2.
Elsewhere in the men's, top seed and Swiss team flag-bearer Roger Federer had an easier time in his match.
He began a bid for his first Olympic medal with a trouble-free victory over Dmitry Tursunov of Russia 6-4 6-2.
The Swiss converted his first break point at 1-1 in the first set and broke twice in the second to win in one hour and five minutes.
Lleyton Hewitt will face Rafael Nadal in the second round
Federer finished fourth in the 2000 Games and will face either Lee Hyung-taik or Rafael Arevalo in the second round.
Nadal, who will take over from Federer as the world number one on 14 August, will face Australia's Lleyton Hewitt in the second round, though he admitted his first match had been difficult.
The third-seeded Novak Djokovic fo Serbia beat Robby Ginepri of the United States 6-4 6-4, and Igor Andreev of Russia defeated Sam Querrey , also of the United States 6-4 6-4.
Meanwhile, David Nalbandian of Argentina disappointed local support with his defeat of Zeng Shaoxuan of China 6-2 6-1.
Chile's double Olympic gold medallist Nicolas Massu launched his defence of his singles' title on Monday with a workmanlike 6-4 7-5 victory over Belgian Steve Darcis.
Massu had to be awarded a special place in the tournament by the International Tennis Federation because his low singles' ranking of 125 prevented automatic qualification.
French 10th seed Gilles Simon went through against Robin Soderling, joining Mikhail Youzhny, Olivier Rochus, Thomas Johansson and Victor Hanescu in the second round.
Former world number one Lleyton Hewitt beat Jonas Bjorkman to set up a clash with Nadal while 10th seed Stanislas Wawrinka and Croatia's Marin Cilic also progressed.
In an upset to form, fifth seed David Ferrer of Spain was knocked out by Janko Tipsarevic 7-6 (10/8) 6-2 and Britain's Andy Murray slumped to a 6-7 (5/7) 4-6 defeat by Lu Yen-Hsun of Chinese Taipei.
Serena Williams makes her debut in the Olympic women's singles
In the women's singles, number four seed Serena Williams moved into the second round after a rain-affected straight sets win over tournament debutant Olga Govortsova of Belarus.
The eight-time Grand Slam champion led 47th-ranked Govortsova 6-3 2-1 on Sunday before rain forced an end to play.
Resuming on centre court on Monday, the American broke the Belarusian twice and held serve to see out the second set 6-1.
Her sister and seven seed Venus Williams also progressed by beating Switzerland's Timea Bacsinszky 6-3 6-2 in her first match since winning her Wimbledon title.
In other women's matches, Russian pair Dinara Safina, the sixth seed, and ninth-seeded Vera Zvonareva beat Mara Santangelo of Italy and China's Yan Zi respectively.
Slovakia's Daniela Hantuchova saw off Japan's Ai Sugiyama 6-2 7-5, while number 15 seed Alize Cornet of France fought back from a set down overnight for a 4-6 6-1 6-4 win over the Czech Republic's Nicole Vaidisova.
Spain's Maria Jose Martinez Sanchez thrashed Australian Alicia Molik 6-1 6-1, while China's Peng Shuai outplayed Spaniard Carla Suarez Navarro 7-5 7-6 (7/2).