Weather-related delays -- both for heat and rain -- pushed back play and the del Potro-Bautista-Agut pairing was one of several that ended after midnight Thursday.
Tournament organizers invoked extreme-heat policies and play was stopped on some courts as temperatures topped out at 108 degrees Fahrenheit Thursday. Another day of 100-degree-plus temperatures are predicted before the heat wave breaks Saturday.
Del Potro seemed in control early in the match but Bautista Agut battled back for a 4-6, 6-3, 5-7, 6-4, 7-5 win in 3 hours, 53 minutes. It was the first time Bautista Agut defeated a Top 10 player. He had five breaks, including the only break of the fifth set, to get to the third round of a major tournament for the first time in his career.
Roger Federer became the first player to win 70 Australian Open matches with a victory Thursday that put him in to the third round. Federer is a four-time Australian Open champion and has been a semifinalist five times and a runner-up another time.
Federer was sharp in getting No. 70, winning more than 70 percent of the points on serve and dropping his serve once in a 6-2, 6-1, 7-6 (7-4) decision over Blaz Kavcic. Federer had 11 aces among his 52 winners and finished with six breaks.
World No. 1 Rafael Nadal took out local favorite Thanasi Kokkinakis 6-2, 6-4, 6-2. He didn't lose his serve while taking more than three-quarters of the points when serving.
Andy Murray, seeded fourth and last year's runner-up, was rolling along with a two-set lead but found himself down 5-1 in the third set to Vincent Millot. Murray, however, won the last 23 points and six straight games for a 6-2, 6-2, 7-5 victory.
The final second-round match ended about 1:40 a.m. when 27th-seeded Benoit Paire rallied for a 6-7 (5-7), 6-7 (5-7), 6-4, 6-2, 6-2 win over Australian teenager Nick Kyrgios.
No. 10-seeded Jo-Wilfried Tsonga got by Thomaz Bellucci 7-6 (8-6), 6-4, 6-4 while 11th-seeded Milos Raonic defeated Victor Hanescu 7-6 (11-9), 6-4, 6-4 and No. 16 Kei Nishikori beat Dusan Lajovic 6-1, 6-1 7-6 (7-3).
Donald Young ousted 24th-seeded Andreas Seppi 6-4, 2-6, 6-3, 4-6, 7-5 with two of his six breaks coming in the 50-minute fifth set. Teymuraz Gabashvili also went the distance before eliminating 31st-seed Fernando Verdasco 7-6 (7-1), 3-6, 2-6, 6-4, 6-4.
No. 25-seeded Gail Monfils topped Jack Sock 7-6 (7-2), 7-5, 6-2 and will meet Nadal in the third round.
Other players advancing included No. 18-seeded Gilles Simon, 22nd-seeded Grigor Dimitrov and No. 26 Feliciano Lopez while additional unseeded players still in the draw include Stephane Robert and Martin Klizan.