Juan Martin del Potro's comeback from injury briefly stalled with a 6-2, 7-5 loss to Germany's Florian Mayer in the second round of the Sydney International on Wednesday.
After beating Feliciano Lopez in his comeback match in the first round on Monday, the 2009 U.S. Open champion lost to the unseeded Mayer in 74 minutes. Del Potro dropped his serve three times in the match including, crucially, at 5-5 in the second set.
Mayer said it was clear del Potro was not yet 100 percent after a wrist injury last year.
''His serve is really strong but from the baseline he missed a lot,'' Mayer said. ''I think also he was maybe a little bit tired from his long match Monday because when you don't play for such a long time and you play your first match over three hours, it's really tough.''
Before his win Monday, del Potro's last tour-level victory came against Mayer in the third round of the 2010 Australian Open. Del Potro was sidelined for most of last season, causing him to drop out of the world's top 200.
''Of course I want to play and win a tournament,'' del Potro said. ''When you lose, it's hard. But now it's different. I have different sensations. I lost, but I have another tournament in three days. I'm in good shape. I have my wrist in good shape. That's good.''
In other matches Wednesday, No. 3-seeded Ernests Gulbis of Latvia beat Igor Andreev of Russia 2-6, 6-4, 6-3.
The world No. 24, who lost to Rafael Nadal in the quarterfinals of last week's Qatar Open, lost his serve twice in the opening set. But he broke Andreev once in each of the last two sets and dropped only five points on serve in the final set, finishing with 17 aces.
Gulbis' quarterfinal opponent will be Ukrainian Sergiy Stakhovsky, who beat No. 7-seeded Guillermo Garcia-Lopez of Spain 6-3, 6-4.
Last year's finalist and No. 5-seeded Richard Gasquet came back from a set down to beat Italian Andreas Seppi 3-6, 7-5, 6-4 in 2 hours, 4 minutes. Gasquet next meets fourth-seeded Serbian Viktor Troicki, who beat former finalist Juan Ignacio Chela of Argentina 6-2, 6-3.
Frenchman Gilles Simon earned a quarterfinal spot by winning two matches on Wednesday. He completed a 6-0, 6-3 win over No. 8-seeded Yen-hsun Lu, in a rain-delayed match, them beat Portugal's Frederico Gil 6-1, 6-3.
In the women's quarterfinals, eighth-seeded Li Na of China beat Svetlana Kuznetsova 3-6, 7-6 (4), 6-3, rallying from 6-3, 5-3 down.
''I didn't want to have any regrets after the match so I just tried to calm down,'' Li said.
She'll meet Bojana Jovanovski, the Serbian qualifier who beat Flavia Pennetta of Italy 6-4, 6-1 to reach her first WTA semifinals.
No. 3-seeded Kim Clijsters beat No. 7 Victoria Azarenka 6-3, 6-2 in 85 minutes to advance to a semifinal against Alisa Kleybanova of Russia. Kleybanova topped Dominika Cibulkova of Slovakia 6-3, 5-7, 6-2.
