Del Potro falls in Sydney
Juan Martin del Potro admits he is still well off his best as his comeback hit a setback after defeat to Florian Mayer at the Sydney International.
Having claimed his first victory in nearly a year on Monday, del Potro failed to string a second straight win together, after falling 6-2 7-5 to the German world No. 37.
Del Potro, who has slumped outside of the world's top 250 following a serious wrist injury, but has been granted a wildcard for the Australian Open next week. But despite the lack of match practice - playing just two matches ahead of the first major of the year, the Argentine insists he is happy with his progress.
"When you lose, it's hard, but now it's different, I have different sensations," del Potro said. "I lost but I have another tournament in three days. I have my wrist in good shape. That's good.
"Mayer played better than me but it's normal. I am very far off my good game but I did my best. It was shorter, but I tried."
Next up for Mayer will be Potito Starace, who booked his place in the last eight with a 7-6(5) 2-6 6-3 win over Lukasz Kubot.
Top seed Sam Querrey's preparations for the Australian Open were dented when he fell 6-4 6-3 to Alexandr Dolgopolov. The Ukrainian made real inroads into Querrey's second serve as he booked his place in the quarter-finals, where he will meet Gilles Simon, who beat Portuguese qualifier Frederico Gil 6-1 6-3.
Third seed Ernests Gulbis battled back from a set down to beat Igor Andreev 2-6 6-4 6-3, while Richard Gasquet encountered a similar challenge in the guise of Andreas Seppi. The No. 5 seed lost the opening set, but held his nerve to progress with a 3-6 7-5 6-4. He will now face fourth seed Viktor Troicki, who overcame Juan Ignacio Chela 6-2 6-3.
Meanwhile, at the Heineken Open in Auckland, top seed David Ferrer survived a wobble to progress to the last eight with a 3-6 7-6(1) 6-4 victory over Germany's Tobias Kamke.
Sixth seed David Nalbandian set up a mouthwatering quarter-final clash against third seed John Isner after both players won their respective matches on Tuesday.
French qualifier Adrian Mannarino's impressive run of form continued, reaching the last eight with a 7-5 6-4 win over compatriot Arnaud Clement. Santiago Giraldo upset fourth seed Albert Montanes to set up a quarter-final clash with Thomaz Belluci.