García-López started playing tennis at the age of 7 and turned pro in 2002. His favorite surface is sand, for his best shot he holds the backhand along the line (longline).
After turning pro in 2002, he made his breakthrough in 2005. The Spaniard surprisingly reached the semi-finals at the beginning of the season in Chennai , where he had to admit defeat to eventual winner Carlos Moyá 4: 6, 6: 2, 6: 7. Also in the semi-finals in Estoril he only lost against the eventual tournament winner Gastón Gaudio 4: 6, 6: 2, 2: 6. García-López reached the quarter-finals in St. Pölten . In 2006 he reached the quarter-finals in Delray Beach , Valencia , Bucharest and Estoril, where he lost 6-7, 4-6 to Moyá. He finished the year in 69th place in the world rankings.
In 2007 the Spaniard reached the quarter-finals again in Delray Beach, but had absolutely no chance there against Nicolás Massú . In Indian Wells García-López made it to the third round and in Monte Carlo he was also a lucky loser in round three, where he could not use three match points against Philipp Kohlschreiber . In Estoril he reached the quarter-finals and lost against the top ten player Novak Đoković 4: 6, 6: 1, 5: 7. At the French Open he defeated the ten seeded Tomáš Berdych and at the grass tournament in Nottingham he surprisingly made it to the quarter-finals. In doubles, the Spaniard reached the finals at the Weissenhof in Stuttgart , where he lost in two sets with Fernando Verdasco .
In his first tournament of the 2008 season in Chennai García-López reached the quarterfinals, where he was defeated by his top-seeded compatriot Rafael Nadal . At the Australian Open he surprisingly defeated 18 seed Juan Ignacio Chela in four sets and moved into the third round. At the tournament in San José at the end of February, the Spaniard unexpectedly reached the semi-finals after a 7: 6, 6: 7, 7: 6 over the service giant John Isner . Against the top seeded Andy Roddick , he was eliminated 6: 7, 1: 6. In the world rankings, he improved to 60th place. After a disappointing clay court season, García-López caused a sensation at Wimbledon . He threw the 19th seed Nicolás Almagro out of the tournament and was only eliminated in round three against the later semi-finalist Rainer Schüttler . Back on the sand, he reached the semi-finals in Gstaad at the beginning of July after defeating the third seeded Michail Juschny . There he was defeated 6: 7, 7: 5, 6: 7 to the Russian Igor Andreev after a hard fight , with the match dragging on for two days due to interruptions in the rain. He finished the year in 62nd place.
In 2009 García-López reached the quarterfinals in Johannesburg , Delray Beach and Houston . Shortly before the French Open , he surprisingly won his first ATP title at the tournament in Kitzbühel and made it into the top 50 of the world rankings for the first time. In Paris, however, he failed in round one to Andreas Seppi . Somewhat surprisingly, the clay court specialist reached the semi-finals at the grass tournament in Eastbourne . At Wimbledon, world number one Roger Federer was the last stop in round two and then the Spaniard went back to the sand. He reached the third round in Hamburg as well as in the hard court tournament in Cincinnati after victories over Michail Juschny and top ten player Fernando Verdasco. At the end of the year he reached the quarter-finals in Stockholm and Valencia and finished the season in the top 50.
At the beginning of the 2010 season, Guillermo García López won his first double title in Doha with his compatriot Albert Montañés . In singles he reached the last sixteen after five first round defeats in a row in Indian Wells after defeating top 10 player Marin Čilić . He also reached the round of 16 at the Masters tournaments in Rome and Madrid . In June 2010 García-López was in Eastbourne for the second time in an ATP final, in which he was defeated by the French Michaël Llodra in two sets. He then achieved his best world rankings to date with 38th place. In the following three months he could only win two ATP matches, but then he achieved a sensation in October 2010 at the Thailand Open in Bangkok: After victories over the seeded Michael Berrer and Ernests Gulbis , the Spaniard met world number one Rafael Nadal in the semifinals which he defeated in three sets. In the final, he then defeated Jarkko Nieminen and thus secured his second ATP individual title. His strong form in Asia continued, at the master's tournament in Shanghai defeated the top players Andy Roddick and Tomáš Berdych, who were seeded at 10 and 7 respectively , and reached the quarter-finals. After these successes, García-López was able to penetrate the top 30 for the first time, but after two opening defeats in Vienna and Valencia he lost points again and ended the season in 33rd position.
In the 2011 season he could no longer build on the strong form of autumn, at the Australian Open he reached the third round and failed there at the later finalist Andy Murray . After reaching the semi-finals in Zagreb , he rose again in the ranking and reached his previous career high with position 23 on February 21, but there were no further successes, despite good performances, a series of first and second round defeats followed, only in Madrid García-López was able to celebrate small successes with a round of 16 and in Roland Garros with reaching the third round.